Computational quantum chemistry is a useful tool to assess the utility of novel organic semiconductors. While these characterizations are often valuable for the specific systems under investigation, their broader impact tends to be more limited.
Our work focuses on merging computational chemistry and molecular modeling with Big Data ideas. The virtual high-throughput screening of compound libraries allows us to generate huge quantum chemical data sets, which can be employed for data-driven discovery. To extract an understanding of the underlying structure-property relationships from these data sets we develop chemical data mining tools. These have their roots in machine learning, statistical learning, and information science. We will discuss the utility of these tools and the resulting models for the prediction of molecular properties without the need for expensive quantum chemical calculations.

Numerous sources project the carbon-based, printable, and flexible electronics industry to have a multibillion dollar economic impact in the next several years, with influences on industries such as lighting, displays, sensing, energy conversion and storage, medical diagnostics, biocompatible electronics, and environmental monitoring among others. Computational studies have certainly enlightened our understanding of the materials on which organic devices are based, especially regarding molecular and band structures, and film deposition. Still, given their anticipated importance, the emerging computational infrastructure being promoted by "Materials Genome Initiative" actions has been underutilized for these technologies. There are numerous fundamental questions, many being discussed in this symposium, that would benefit from sustained computational efforts. An NSF/DOE/NIST-sponsored workshop in November of 2013 on this topic identified a series of grand challenges concerning structures, processes, properties, and architectures that could be tackled by coordinated computational investigations, as well as some singnificant but addressable barriers that need to be overcome for such efforts to bear fruit. This presentation will summarize these opportunities and issues, and invite discussion of some further computational directions. The contributions of workshop participants to this talk is gratefully acknowledged.

Minimizing charge carrier injection barriers and extraction losses at interfaces between organic semiconductors and metallic electrodes is critical for optimizing the performance of organic (opto-) electronic devices. Here, we present results from a detailed electrostatic model, capable of reproducing the alignment between the electrode Fermi energy and the transport states in the organic semiconductor both qualitatively and quantitatively. Covering the full phenomenological range of interfacial energy level alignment regimes within a single, consistent framework and continuously connecting the limiting cases described by previously proposed models allows us to resolve conflicting views in the literature. Our results highlight the density of states in the organic semiconductor as a key factor. Its shape and, in particular, the energy distribution of electronic states tailing into the fundamental gap is found to determine both the minimum value of practically achievable injection barriers as well as their spatial profile, ranging from abrupt interface dipoles to extended band-bending regions.
As a case study, we will discuss this remarkable transition for pentacene, where ample experimental reference data is available [1]. Additionally, the effect of introducing ultrathin alkali-halide interlayers between metal and organic on the interfacial energy level alignment and its spatial profile will be discussed on the example of C₆₀ fullerene [2].
[1] M. Oehzelt, N. Koch, G. Heimel, #8222;Organic semiconductor density of states controls the energy level alignment at electrode interfaces”, Nat. Comm. 5, DOI: ncomm5174 (2014).
[2] H.Wang, P. Amsalem, G. Heimel, I. Salzmann, N. Koch, M. Oehzelt, #8222;Band-Bending in Organic Semiconductors: the Role of Alkali-Halide Interlayers“, Adv. Mater. 26, 925 (2014).

The charge carrier mobility µ is essential for understanding and improving organic electronic devices. It has an impact on charge transport as well as most likely on exciton dissociation, recombination dynamics, and other processes. A better understanding of the interplay between material structure, processing conditions, and the mobility opens the road for an improved understanding of transport and a targeted design of optimized molecules and devices.
In this contribution, we show how the charge carrier mobility µ can be directly measured in vertical device geometry as a function of the electric field F and the charge carrier density n, yielding the mobility function µ(F, n) for electrons and holes separately using electron-only or hole-only devices. The measurement is based on a new evaluation strategy for space-charge limited currents in single carrier devices: Spatial potential mapping (POEM) by varying the device thickness allows for a novel approach to fully and quantitatively characterize charge transport. The obtained mobility µ(F, n) is the average effective mobility of free and trapped charges together, which is a realistic description in energetically disordered materials where the differentiation between traps and mobile sites is often difficult or arbitrary. The effective mobility µ(F, n) is obtained in a model-free approach and allows for an unbiased interpretation of the mechanisms determining the transport mechanism. With this approach, the measurement is applicable to a wide range of materials including disordered organic materials and material blends, and complex morphologies.
Mobility measurements are performed on neat organic materials as well as donor-acceptor blends for photovoltaics, comprising small molecule organic semiconductors (e.g. high-efficiency oligothiophene derivatives or phthalocyanines). The effects of structural modifications and varying processing conditions (e.g. substrate temperature), and of varying the blend ratio, including highly diluted blends with small donor concentration down to 1%, are investigated. Generally, a pronounced field activation is observed, and in several materials also a charge density dependence can be resolved. The results allow for an advanced understanding of charge transport in donor-acceptor blends.

The morphology of blends of conjugated polymers (electron donors) and fullerene derivative molecules (electron acceptors) strongly affects the efficiency of organic solar cells. Recently, minority component solvent additives such as diiodooctane (DIO) and chloronapthalene (CN) have been used in acceptor-donor blends to alter blend morphology and to significantly improve device efficiency. In this work we present coarse-grained molecular dynamics simulations of conjugated polymer and fullerene derivative blends with additives of varying length, chemical functionalization, and volume fraction to understand how these additives affect morphology and to elucidate what physical and chemical features of solvent additives are important for tuning morphology.

We develop theoretical descriptions for charge transport in organic semiconductors. For the localized charges, we found the quantum nuclear tunneling effect is essential which could manifest isotope effect for mobility as well as exotic optical feature. For the bandlike transport, we propose a Wanner extrapolation scheme for computing the electron-phonon interaction matrix for the Boltzmann equation. Most interestingly, we applied the time-dependent wavepacket diffusion method for describing polaron motion with arbitrary localization to study the carrier dynamics in BTBT system.

In organic thin-film transistors (OTFTs), charge injection barrier originating from a Schottky contact formed at a metal/organic interface is one of the most important factors that govern the device performance. Therefore, in device design based on realistic OTFTs, Schottky contact should be considered for numerical investigation of OTFTs by employing an appropriate physical model. In this study, a thermionic field emission (TFE) model, which accounts for tunneling of thermally excited electrons at a reverse-biased Schottky contact, has been newly utilized as charge injection model in OTFTs. Here the difference in the effects of Schottky barrier and contact-area-limited doping for device configuration in OTFTs are discussed. In addition, the comparison between experimental current-voltage curves and simulation results in OTFTs with Schottky contact was performed. Numerical investigation of OTFTs was carried out with a Thin-film Organic Transistor Advanced Simulator (TOTAS), which was originally developed by our group. The TFE model was utilized as the boundary condition for the current continuity equations at the source electrode/semiconductor interface. Typical electrical characteristics in realistic OTFTs, such as nonlinear behaviors in output characteristics, can be reproduced with this simulation based on tunneling injection. Our simulation results also reveal that bottom-gate, bottom-contact configuration is affected by Schottky barrier more severely than bottom-gate, top-contact one under the same condition for device parameters, and that this discrepancy in device characteristics can be completely eliminated by fabricating highly-doped semiconducting layers in the neighborhood of contact electrodes, which is called contact-area-limited doping. Besides, the existence of intrinsic Schottky barrier is suggested even though an ohmic-contact condition is assumed, which becomes more prominent for lower carrier concentration in an active semiconductor layer. The Schottky barrier height and the gate-voltage dependence of the carrier mobility in a pentacene TFT were also properly evaluated from the experimental results by utilizing the device simulation. In conclusion, device simulation for OTFTs with the thermionic field emission model was demonstrated for the first time. The TFE model is an appropriate and useful model for understanding operation mechanisms, characterizing carrier injection and transport properties, and device design of OTFTs. The study also proves availability of the contact-area-limited doping for fabricating high-performance OTFT devices.

Vertical organic field-effect transistors (VOFETs) are a novel type of organic semiconductor devices, the first of such devices having been presented by Ma et al. in 2004 [1]. The idea of this novel device concept is to overcome the limitations often faced in conventional organic field-effect transistors (OFETs), where performance parameters (e.g. cut-off frequency or transconductance) are limited by the channel length of the OFET. The VOFET concept developed at IAPP [2] allows for downscaling of this channel length to the order of nanometres, while using a novel photolithography approach [3] to fabricate the patterned source electrode required for VOFET operation. As was shown in our previous publication [2], such VOFETs can suffer from so-called short-channel effects. In particular injection from the source contact into the semiconductor can become a limiting factor for the device performance of short-channel VOFETs.
The work presented here therefore investigates the effects of so-called contact doping, a concept already successfully employed in conventional OFETs in order to enhance charge carrier injection into the semiconductor and reduce contact resistance [4-6]. It was possible to show that the organic p-dopant C60F36 can act as contact doping layer for Pentacene VOFETs (with vertical channel lengths of only 30nm) and OFETs alike, both as single layer and blended into a Pentacene matrix. In both cases, the contact doping successfully reduces the contact resistance for devices with gold electrodes and even allows for the use of aluminum as electrode material despite the mismatch between the aluminum work function and the Pentacene HOMO.
References
[1] L. Ma et al, Appl. Phys. Lett. 85, 21 (2004)
[2] H. Kleemann et al, Small9, 21 (2013)
[3] H. Kleemann et al, Org. Elec.13, 3 (2012)
[4] T. Matsumoto et al, Org. Elec.14, 10 (2013)
[5] B. Lüssem et al, Nat. Comm.4 (2013)
[6] Z. Wang et al, Appl. Phys. Lett. 100, 4 (2012)

A key challenge is to create computational methods that can properly handle charge transport in organic electronics and particularly solar cells, bridging across multiple length and time scales, in the presence of defects, traps, impurities, disorder, and particularly variations in nanomorphology. Our research has focused particularly on developing high-performance Monte Carlo charge transport simulations that explicitly treat disorder and defects of various kinds. Charge transport parameters are, where possible, derived from first-principals calculations, particularly the charge delocalization length. Our simulations are validated against in-house and external experimental measurements and have reproduced several non-obvious effects, including a negative differential resistance (NDR) effect in organic semiconductor mixtures. We have also developed a delocalized charge model capable of classically treating accurate electrostatic interactions in organic solar cells. Using these charge transport simulations, we are currently performing inverse design of optimal morphologies for organic solar cells. We find that traditional columnar "comb" designs may perform worse than conventional bulk heterojunction morphologies. Progress towards identifying important structure-property relationships for optimal solar cell efficiencies will be discussed, as well as directions for synthetic and experimental targets.

The molecular weight (MW) of a conjugated polymer has been recognized to influence various physical, photophysical, and electronic properties in the solution and solid states. Here, MW fractionation of a synthesized donor-acceptor conjugated polymer was carried out to obtain two polymer species of different MWs. Then, the low- and high-MW polymeric species were investigated in terms of their intrinsic material properties. Precisely self-assembled polymer nanowire crystals made from the low-MW polymer were employed in field-effect transistors to observe the intrinsic charge transport property. An unprecedentedly high carrier mobility of around 24.0 cm²V^-1s^-1 was obtained. A DPP-based PNW-based phototransistor showed very high photoresponsivity (R = 160 AW^minus;1 at V_G= +4.0 V, R_max ~ 1920 A W^minus;1). The highly ordered single-crystalline DPP-polymer nanowires displayed much better device performance compared with thin-film-based devices.

The charge transport property of highly crystalline semiconducting conjugated polymer synthesized by using diketopyrrolopyrrole (DPP) flanked with thiophene was studied. PTDPP-DTTE polymer, bearing thiophene-flanked DPP and Di(thienothienyl)ethylene (DTTE), exhibited considerably high carrier mobility at thin-film transistors. We then applied two extrinsic treatments to the PTDPP-DTTE polymer, solution shearing (SS) and template-guided solution shearing (TGSS) in order to increase the anisotropy of the polymer chains in order to increase the carrier mobility in the shearing direction. Field-effect transistors composed of micro-patterned prism arrays of PTDPP-DTTE displayed the highest mobility of approximately 7.3 cm² V^-1 s^-1, which is higher than that obtained from TFTs treated under the solution-shearing process. Therefore, the TGSS method is recognized as a very promising method to increase the performance of electronic devices requiring high anisotropy of the conjugated molecules and polymers.

Recently, the charge-transport phenomena of organic conjugated materials have been intensively investigated because of the potential applications in electronics and optoelectronics. Among them, organic field-effect transistors (OFETs) fabricated from either thin films or well-defined single crystals (SCs) as charge-transporting layers are one of the most promising electronic devices. In particular, the unique anisotropic arrangement of organic semiconducting molecules owing to their strong intermolecular interactions is expected to have a significant influence on the FET performance, because the larger overlap of p-orbitals between neighboring molecules may increase the bandwidth and facilitate charge-transport. In this regards, porphyrin is one of the most important p-conjugated planar molecules and they have often been employed in OFETs, organic phototransistors (OPTs), and organic photovoltaics (OPVs). Because of their unique structure, porphyrins may provide multiple interactions such as hydrogen bonding, π-π stacking, electrostatic interactions, and metal-ligand coordination. However, the performances of recently developed porphyrin-based OFET devices show relatively low carrier mobilities. A deeper understanding of such systems has hardly been achieved because of the lack of information on the molecular packing and intermolecular arrangement (which are closely related to the OFET performance), since most porphyrin-based OFET devices are based on thin films or polycrystalline objects prepared by spin-coating or vacuum-deposition processes and showing relatively poor device performances because of the limitation in use of π-conjugation.
For a high degree of crystallinity with an excellent determinacy as well as a high corresponding device performance, the extension of p-orbitals and the location of conjugative substituents on the porphyrin core play major roles. Therefore, I like to present a series of new p-extended porphyrin derivatives and their OFET devices performances along with films and single-crystals.
References
1. S. Choi, S. H. Chae, M. H. Hoang, K. H. Kim, J. A. Huh, Y. Kim, S.-J. Kim, D. H. Choi, S. J. Lee, Chem. Eur. J. 2013, 19, 2247.
2. S. Choi, S. H. Chae, J. Shin, Y. Kim, S.-J. Kim, D. H. Choi, S. J. Lee, Chem. Comm. 2013, 49, 3994.
3. M. H. Hoang, Y. Kim, M. Kim, K. H. Kim, T. W. Lee, D. N. Nguyen, S.-J. Kim, K. Lee, S. J. Lee, D. H. Choi, Adv. Mater. 2012, 24, 5363.
4. D. H. Lee, S. Kim, M. Y. Hyun, J.-Y. Hong, S. Huh, C. Kim, S. J. Lee, Chem. Comm. 2012, 48, 5512.
5. K. H. Kim, S. Y. Bae, Y. S. Kim, J. A. Hur, M. H. Hoang, T. W. Lee, M. J. Cho, Y. Kim, M. Kim, S.-J. Kim, K. Lee, S. J. Lee, D. H. Choi, Adv. Mater. 2011, 23, 3095.

A cross-coupling reaction between dihaloarylene monomers and unsubstituted arylene monomers in the presence of a palladium catalyst, the so-called palladium-catalyzed direct arylation, has been paid numerous attention as an environmentally-friendly, efficient, and low-cost method for the synthesis of conjugated polymers compared to conventional cross-coupling reactions, such as Stille coupling reaction and Suzuki coupling reaction. In recent years, a variety of conjugated donor polymers have been synthesized by the palladium-catalyzed direct arylation. On the other hand, the synthesis of conjugated acceptor polymers by the palladium-catalyzed direct arylation is far less developed. The first example of the synthesis of conjugated acceptor polymers by the palladium-catalyzed direct arylation was reported in 2012 by Horie and co-workers; a naphthalene bisimide (NBI)-based acceptor polymer, a potential acceptor polymer for organic photovoltaics and organic field-effect transistors, was synthesized using conventional NBI-based dibromo monomer under the palladium-catalyzed direct arylation conditions. However, the molecular weight of the obtained NBI-based acceptor polymer was extremely low (M_n ~ 2000). Therefore, the development of conjugated acceptor polymers with high molecular weights by the palladium-catalyzed direct arylation remains a big challenge.
We herein found an efficient palladium-catalyzed direct arylation methodology for the synthesis of the NBI-based acceptor polymer (PNBI3T-Dr) using a newly designed thiophene-extended NBI monomer under the optimal palladium-catalyzed direct arylation conditions, which yielded PNBI3T-Dr with a high molecular weight (M_n > 30000, and M_w ~ 90000). To the best of our knowledge, our work is the first example for the efficient synthesis of conjugated acceptor polymers by the palladium-catalyzed direct arylation. NMR spectroscopy and optoelectrochemical properties of PNBI3T-Dr indicated that there were no structural defects on the NBI-based acceptor polymer synthesized by the palladium-catalyzed direct arylation. These results revealed that the palladium-catalyzed direct arylation could be an alternative synthetic method even for conjugated acceptor polymers. Furthermore, the photovoltaic performances using PNBI3T-Dr are also going to be presented in the symposium.

Conjugated block copolymers have the potential to tune donor/acceptor interfaces and the mesoscale structure within the active layer of organic photovoltaic devices. The ability to control and modify the micro-phase separation of the copolymer can offer a useful platform in understand the relationship between chemical structure, nanoscale morphology, and photovoltaic device performance. We have demonstrated that utilizing poly(3-hexylthiophene)minus;blockminus;poly-((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(thiophen-5-yl)-2,1,3-benzothiadiazole]-2prime;,2"-diyl) (P3HT-b-PFTBT) as the active layer of solar cells leads to power conversion efficiencies near 3% and remarkable 1.2 V open-circuit voltages without the use of a fullerene acceptor. We also find that we can control the P3HT crystallite orientation from “face-on” (π-stacking predominantly out-of-plane) to “edge-on” (π-stacking mainly in-plane) by altering the solvent casting conditions. After optimizing fabrication for each set of devices separately, we find that our device performance is independent of the crystallite orientation, contrary to our current understanding - face-on crystal orientations are expected to maximize charge transport in solar cells. We hypothesize that the ubiquitous broad distribution of crystallite textures always provides pathways for charge extraction, regardless of the average orientation of P3HT crystals.

#12288;#12288;#12288;Organic field-effect transistors (OFETs) have been of considerable interest, because of their utilities as, for example, flexible flat-panel displays, electronic paper, and chemical sensors. OFETs of high performance have been often achieved by using expanded heteroaromatic p-systems including thiophene oligomers, and acene molecules like pentacene derivatives, as the semiconducting materials. However, the values of field-effect mobility, mu;, have been still lower than the one for inorganic MOS FETs. To increase the carrier mobility, much research has been focused on organic materials that expansion of π stacking and planar conjugated structures. #12288;#12288;#12288;In this study, we investigated the characterization of thin film OFETs based on newly developed compounds, picene and 3,10-disubstituted picene derivatives. The aromatic hydrocarbon of picene possesses the same number of benzene rings as pentacene, and the expanded and delocalized π-system exists within the molecule. The fabrication of these thin films were demonstrated by spin-coating and vapor deposition process. At spin-coating, it was not easy to fabricate uniform thin films of picene and picene derivatives at a room temperature, and they always exhibited inhomogeneous surface morphology because these molecules were recrystallized on the substrate during evaporation of organic solvent. At vapor deposition, we used polymer-treated SiO₂ dielectric layer to examine the effect of polymer coating. Polymer coating of the SiO₂ gate dielectric can be easily accomplished by a spin-coating technique under ambient conditions. Field-effect characteristics were strongly influenced by the applied polymer insulators, and the best field-effect mobility(5.2cm²V^-1s^-1) was obtained for the CYTOP-treated device incorporating 3,10-diethylpicene.

Organic semiconductors have been currently focused on the field of material sciences, because they are promising candidates for use in flexible, large-area and low-cost electronic devices such as flexible displays, radio frequency identification tags and flexible sensors. In particular, organic complementary devices are desired to be developed for extensive applications to organic logic circuits, where high performance organic field-effect transistors (OFETs). To the fabrication of electronic devices such as OFETs, the use of well-ordered crystalline thin films and well-defined single-crystals are promising for the fabrication of charge-transporting layers. In this regard, the solubility of organic semiconductors has been one of the most important factors and focused on the development of highly soluble materials, because they should lead to scalable and practical low-cost devices. In this work, we focused on dialkyl substituted DNTT derivatives, as solution-processible organic semiconductors, in which two solubilizing long alkyl groups are introduced in the long-axis direction for the molecular core. The drop-cast method was used to produce the organic single-crystal ribbons. We used an n⁺-doped Si wafer with 220 nm thickness thermally grown SiO₂, which was covered with a thin film (ca. 30 nm) of poly(methyl methacrylate) (PMMA) insulator. The solution of DNTT derivatives was dropped onto the substrate, and was allowed to crystallize under air atmosphere. Source and drain electrodes were painted with a colloidal graphite paste. The field-effect mobility (mu;) of the OFET devices were measured under vacuum from the saturation regime according to the equation: I_d=mu;C_i(W/2L)(V_g-V_th)², where C_i is capacitance of the insulator, V_d and V_th are the gate and threshold voltages, respectively. The DNTT derivatives based organic transistors showed typical p-channel FET characteristics. At present, the best field-effect mobility of 2.5cm²V^-1s^-1 with current on/off ratio of 10⁵was obtained for 6,6&’-8P-21DNTT based single-crystal transistors.

We investigate theoretically the switching behavior of a dithiolated phenylene-vinylene oligomer sandwiched between Au(111) electrodes using self-interaction corrected density-functional theory combined with the nonequilibrium Green's-function method for quantum transport. The molecule presents a configurational bistability, which can be exploited in constructing molecular memories, switches, and sensors. We find that protonation of the terminating thiol groups is at the origin of the change in conductance. H bonding at the thiol group weakens the S-Au bond and reduces by about one order of magnitude the transmission coefficient at the Fermi level, and thus the linear response conductance. Furthermore, protonation downshifts in energy the position of the highest occupied molecular orbital, so that the current of the protonated species is lower than that of the unprotonated one along the entire bias range investigated, from minus;1.5 to 1.5 V. A second protonation at the opposite thiol group has only minor effects and no further drastic reduction in transmission takes place. Our results allow us to re-interpret the experimental data originally attributing the conductance reduction to H dissociation. For further details see Phys. Rev. B. 88, 085438 (2013).

The current-voltage characteristics of self-assembled conjugated molecular wires were studied with conductive probe atomic force microscopy (CP-AFM). Molecular wires where grown using thiophene as the donor unit, and were extensively characterized with infrared spectroscopy, spectroscopic ellipsometry, x-ray photoelectron spectroscopy, cyclic voltammetry, UV-vis spectroscopy and CP-AFM. Molecular junctions were formed by contacting self-assembled monolayers of the wires bound to an atomically flat gold substrate with a conductive AFM tip. This technique yields reproducible formation of nanoscale junctions (~100 nm²) of highly oriented, monodisperse oligomers with a well-defined metal-molecule interface. By systematically increasing the wire length one molecule at a time via aryl imine click-chemistry and measuring the junction resistance we were able to extract the structure dependent tunneling attenuation factor (β), and observe a transition from tunneling to hopping charge transport at a molecular length of 4 nm. The activation energy for hopping through the long wires was extracted from Arrhenius plots of the variable temperature resistance measurements. Incorporating thiophene donor molecules into an oligophenyleneimine wire resulted in a significant increase in both tunneling and hopping resistance with similar attenuation factor values (β) compared to similar wires containing only phenylene units. This suggests that not only size and orientation, but also molecular architecture greatly influences charge transport in nanoscale electronic devices. Combining the step-wise growth method with CP-AFM measurements opens possibilities to further explore the fundamental structure-property relationships for charge hopping through molecular junctions.

Organic field-effect transistors (OFETs) have received significant attention because of the potential use in flexible and/or disposable portable electronic devices. The charge carrier mobility is strongly influenced by the organic semiconductor layer. But, it has been still lower than the charge carrier mobility for inorganic MOS FETs. Therefore, great efforts have been made toward the development of various semiconducting small molecules and polymers. Among these, fused π-conjugated semiconductors such as acene, thienoacene, and thienothiophene-based compounds are promising materials for high-performance OFETs.
In this study we investigated the characterization of thin film OFETs based on newly developed compound, asymmetric chrysene derivatives which regard phenyl chrysene as the main structure. It is expected that the large and planar chrysene core would provide intermolecular interactions to achieve high charge carrier mobilities, and asymmetric molecular structure are promising for improvement of solubility.
Thin films are fabricated by spin-coat and vapor deposition method on a n-doped Si wafer with a 220 nm thermally grown SiO₂. At spin-coating, an organic film are fabricated by using a 0.4 wt% solution in toluene at 2000 rpm for 30 sec. At vapor deposition, the insulating SiO₂ layer was treated with a thin film (ca. 30 nm) of a polymer insulator such as polystyrene (PS) or CYTOP (Asahi Glass Corp.). Molecular packing and orientation was chracterized by atomic force microscope (AFM), and x-ray diffractions (XRDs) analysis.
Organic transistors based on asymmetric chrysene derivatives showed typical p-channel FET characteristics. The field-effect carrier mobility of spin-coated film were determined to be 5.9×10^-4cm²V^-1s^-1 and on/off ratio of 10⁵. Characteristics of vapor deposition film also strongly correlated with the applied polymer insulator, and the best field-effect mobility (3.1 cm²V^-1s^-1 and on/off ratio of 10⁴) was obtained for the CYTOP-treated device.

A spin-coating process to leaded single-crystal organic nanowires arrays and patterning using SU-8 photoresist trenches-assisted self-assembly method that enables the simultaneous synthesis, alignment, and patterning of nanowires from molecular solutions is reported. In our method, the solutions are guided to the edge of the photoresist by the spin-coating, and the solutions are sticked (or pinned) in this position, simultaneously the single crystal organic nanowires can easily be synthesized by self-assembly and crystallization of organic molecules at the edge of photoresist. Using this method, large-area (wafer substrates) single-crystal organic nanowires arrays can easily be obtained, because the spin-coating and photolithography are suitable for convenient, large-area commercial production. This efficient and large-area manufacturing method is used to fabricate high-performance organic nanowire field-effect transistors that exhibit excellent field-effect mobilities (up to 3.2 cm²V^-1s^-1) and the high mobility of device can be used as driving transistor for a LED. Hence, our method facilitates fabrication of high-quality organic semiconductor nanocrystals for fundamental studies, and large-area practical application.

In organic electronic devices charge carrier injection at the metal-organic interface is a limiting factor for device performance [1]. Misalignment of the energy levels at this interface causes a barrier that can be overcome by applying an injection layer between the metal contact and the organic semiconductor. Self-assembled monolayers (SAMs) on the metal surfaces can be used to build up an interfacial dipole thus raising or lowering the metal&’s work function. This allows for adjusting the energy levels to the organic semiconductor in order to improve charge carrier injection [2].
Solution processing of SAMs is crucial for the printability of devices. The formation and ordering of such a SAM are influenced by several factors including immersion time of the substrates, concentration of the molecules in solution, cleanliness of substrate and solution itself.
Infrared reflection-absorption spectroscopy (IRRAS) of SAMs on metal surfaces is very sensitive to the molecules&’ orientation, therefore providing information about the formation process and also the quality of the SAM. Here we present our IRRAS investigation of a perfluorinated alkanethiol SAM, namely 1H,1H,2H,2H-perfluorodecanethiol (PFDT). In addition to IRRAS measurements photoelectron spectroscopy (PES) was performed to analyze the work function shift and chemical composition of the monolayer. Goal of our investigation was to find out if a functional SAM with the expected work function shift forms already on timescales relevant for printing processes.
For that, we varied immersion time and concentration of the PFDT solution to correlate exposure (concentration times immersion time) with SAM formation and functionality. Our findings demonstrate that functionality couples strongly with coverage and is influenced by the orientation process of the molecules. SAMs prepared under realistic printing conditions show that a fully functional monolayer is produced for very short immersion times in the range of seconds revealing the possibility of printing. For immersion times in the range of hours, however, SAM functionality is decreased due to degradation of the PFDT molecules in the monolayer.
[1] Natali, D. & Caironi, M. Charge Injection in Solution-Processed Organic Field-Effect Transistors: Physics, Models and Characterization Methods. Adv. Mater.24, 1357-1387 (2012).
[2] Cheng, X. et al. Controlling Electron and Hole Charge Injection in Ambipolar Organic Field-Effect Transistors by Self-Assembled Monolayers. Adv. Funct. Mater.19, 2407-2415 (2009).

Solution-based methods are attractive for fabricating organic field effect transistors (OFETs). However, it is difficult to achieve highly crystalline thin films of organic semiconductors owing to their preferential three-dimensional growth tendency. Here we report a simple and effective solution method, namely surface tension controlled oriented growth (STOG), to fabricate aligned single crystals of 6, 13-bis(triisopropylsilylethynyl) pentacene (TIPS PEN) on Si/SiO₂ substrate. The STOG method is based on the directed organization of the π-conjugated molecules and does not require any special equipment or post-processing. Besides, the coverage of the highly oriented films on 10x50 mm² Si/SiO₂ substrates is over 70%.
In our approach, substrates with hydrophilic surfaces were immersed in mixed solution and then partially pulled out. Consequently, the surface would be covered with liquid membrane. The mixed solution contained two solvents with different surface tension. As the solvent evaporated, the upper contact line of the liquid membrane spread down the substrate and the lower contact line remained pinned. Finally, ribbon-shaped, centimeter-long, micrometer-wide and highly aligned single-crystalline thin films could be formed on Si/SiO₂ substrates. These single crystals were characterized in details by cross-polarized microscopy and transmission electron microscopy analysis. It was found that both hydrophilic surface and mixed solution were essential to the orientation of the ribbon-shaped crystals. The different surface tension of the two solvents resulted in a recirculation flow in the direction from the upper contact line to the lower contact line. This phenomenon was attributed to the Marangoni effect (surface-tension-driven effect), which arose during the drying processes of the liquid membrane.
The dimensions of the ribbon-shaped TIPS PEN crystals could be controlled and the dimensions used for device application ranging from height of 15-30 nm, width of 30-50 mu;m, and length of 1-3 cm. Under optimum condition, the devices based on the aligned TIPS PEN crystals exhibited average field-effect mobility of 0.8 ± 0.12 cm² V^-1 s^-1 and on/off ratio of 10⁶, which are impressive values for the self-organized π-conjugated molecules derived from solution-processed methods. With regard to stability issue, our devices kept in air over three months showed no obvious degradation. In addition, the influence of crystal thickness and roughness on the charge transport was studied. As the solution concentration varied from 1 to 4 g/L, the thickness of the crystal thin film increased from 16.5 to 76. 2 nm, while the Root Mean Square (RMS)value increased from 0.343 to 1.01 nm, leading to a field-effect mobility declining from 0.8 to 0.035 cm² V^-1 s^-1.
Herein, the STOG method offers a facile route for mass producible fabrication of high-quality semiconductor crystals. Our work paves the way to high-performance, large-area printed electronics.

Donor-acceptor (D-A) semiconducting polymers offer strong intermolecular interactions likely due to the strong local dipoles, resulting in the high charge transport property. The use of strong acceptors, such as naphthalenediimide, diketopyrrolopyrrole, and isoindigo, offers not only strong intermolecular interactions, but also deep LUMO energy levels, which are essential for air stable ambipolar or n-type transistor devices. Thus, the development of strong acceptors is a key issue for the creation of high-performance semiconducting polymers and printable complementary circuit technology.
Recently, we have reported that thieno[3,2-b]thiophene-2,5-dione (TTD) is a promising acceptor unit as polymers with TTD exhibited deep LUMO energy levels below minus;3.8 eV and high carrier mobilities. However, one issue was that the synthesis of a TTD monomer, 3,6-di(5-bromo-4-alkylthiophene-2-yl)thienothiophene-2,5-dione, involved multi-step reactions with a very low total yield (14% / 4steps), which prevented us from the further investigation of TTD-based polymers. In this contribution, we first report on the improved synthesis of the TTD monomer. We also synthesized a series of TTD-based polymers with different thiophene-based donor units to obtain insight into the structure-property relationship in TTD-based polymers for OFET materials.
Compared to the previously reported route to the TTD monomer, the new route afforded the monomer in fewer steps with a higher total yield (40% / 3 steps). Then, we copolymerized it with distannylated ethylene, thienothiophene, bithiophene, naphthodithiophene, and thienylenevinylene monomers to afford a series of TTD-based D-A polymers. The polymers showed deep LUMO energy levels of around minus;3.8 eV regardless of the donor units. On the contrary, HOMO energy levels of the polymers were dependent on the donor units, ranging from minus;5.5 eV to minus;5.1 eV. The similar LUMO energy levels of the polymers are likely due to the fact that the LUMO was localized on the TTD units independently of the donor units as revealed by the DFT calculation. OFET devices based on the polymers exhibited ambipolar characteristics even under ambient conditions. Interestingly, the ratio of electron to hole mobilities of the polymers increased with the decrease of the length of the donor units. The thienothiophene copolymers exhibited the most well-balanced ambipolar characteristics with both electron and hole mobilities over 0.2 cm²V^minus;1s^minus;1.

The palladium-catalyzed Suzuki polycondensation reaction is an indispensable protocol to synthesize fully conjugated organic semiconducting polymers which are employed for organic thin film transistors, light-emitting diodes and organic photovoltaics. In general, di-boronic acids or di-boron esters of electron-sufficient aryl compounds are coupled to aryl di-bromides satisfactor-ily, but this story is no longer applicable to electron-accepting heteroaryl di-boronic acids and di-boron esters. The limited st-ability of boron moieties makes it difficult to synthesize high-molecular weight conjugated polymers from all electron-accepting co-monomers, which must be excellent candidates for electron-transporting organic semiconductors. To resolve this synthetic issue, we proposed a modified Suzuki polycondensation reaction employing “dipotassium heteroaryl bis(trifluoroborate)s” as alternative co-monomers to heteroaryl boronic acids and esters. Using our unique protocol, we were able to synthesize a high molecular weight polymer, P1, from dipotassium bis(trifluoroborate) of 2-alkylbenzotriazole (M1) and dibromobenzothiadiazole (M2) with Li-based inorganic bases. Moreover, we moved forward with other electron-accepting monomers, including dibromo fluorenone (P2), dibromonaphtalene-bis(dicarboximide) (P3) and dibromopyrazine (P4), all of which produced electron accepting semiconducting polymers with moderate to good electron-transporting performance.

Optocouplers are important photoelectric devices with fine anti-jamming ability for noise control in signal communication and detection. Structurally it is composed of a light emitter as the input and a photodetector (PD) as the output. It realizes the function of electrical isolation and signal conversion by coupling electrical input and output ends with optical intermedium. Compared with inorganic optocouplers, considerable attentions have been attracted to organic optoelectronic devices due to the superiorities in low cost, low process temperature, ease of large-area fabrication and flexibility. Organic optocouplers (OOCs) are demonstrated broad application prospects in the field of aerospace.
Based on the organic photodiodes (OPDs) using C₆₀ as the electron-transport material, NPB and m-MTDATA as the hole-transport materials, all small-molecular and high-performance OOCs are fabricated. The dependence of the photosensitive properties of m-MTDATA (NPB) / C₆₀ heterojunction on the light absorption and electric field distribution is analyzed. A model for predicting the trend of photocurrent changed with the structure of the OPDs is set up. We also build devices according to the simulated optimal configurations and gain improved photocurrents as expected.
The maximum light/dark current ratio and current transfer ratio (CTR) of 5 × 10⁴ and 1.3%, were achieved respectively. The cut-off frequency (f_3dB) was 400 kHz, and the output signal was capable to follow the input signal at a modulation frequency of 1 MHz despite of the phase delay, which could be markedly comparable with the commerical inorganic optocouplers. This device could also be used as flexion and mechanical force sensors with the current density changing under different bending conditions due to its flexibility.
Besides, because of its fine electrical isolation ability, we also realized a successful control of the high voltage circuit under 2000 V by a low-voltage-circuit. We foresee that this kind of device could be used in future optoelectronic devices such as intelligent robots to realize isolated analog-to-digital converters and low-voltage-control circuit instead of manual operation in hazardous environments.

To elucidate the relationship between molecule structure and charge transport properties of organic semiconductors is always a main target for researchers in organic electronics. So far great progress has been achieved in understanding the effect of molecule structure on charge transport^{1, 2}. One intriguing question is how the molecule symmetry would affect its electrical transport properties, which is still unclear yet. This question looks especially attracting considering that there have been reports about the improvement of charge carrier mobility by adopting asymmetric molecule structures^{3, 4}.
In our work, we have investigated the influence of the symmetry of the side chain substituents in high-mobility, solution processible n-type molecular semiconductors on the performance of organic field-effect transistors (OFETs). We compare two molecules with the same conjugated core, but either symmetric or asymmetric side chain substituents and investigate the transport properties and thin film growth mode using scanning Kelvin probe microscopy (SKPM) and atomic force microscopy (AFM). We find that asymmetric side chains can induce a favorable two-dimensional growth mode with a bi-layer structure, and the intra-layer charge transport in this film is not obviously disrupted by grain boundaries, which enables ultrathin films with a single bi-layer to exhibit excellent transport properties. On the other hand, the molecule with symmetric side chains adopts an unfavourable three-dimensional growth mode. Kelvin probe measurements show terrace-like features in the potential images of a very thin-film FET device, which means intra-layer transport is severely hindered by high-resistance grain boundaries. To obtain a high performance FET for this molecule, a thick semiconductor film is required for improving the inter-layer transport to help charge carriers avoid going cross grain boundaries. Our results shed light onto the influence of molecule symmetry on charge transport, suggesting that asymmetric side chain substitution provides a powerful approach for molecular semiconductors to adopt a favorable two-dimensional growth mode and to realize high-performance organic FETs with ultrathin active semiconductor layers.
1. I. McCulloch, M. Heeney, M. L. Chabinyc, D. DeLongchamp, R. J. Kline, M. Coelle, W. Duffy, D. Fischer, D. Gundlach, B. Hamadani, R. Hamilton, L. Richter, A. Salleo, M. Shkunov, D. Sporrowe, S. Tierney and W. Zhong, Adv Mater 21 (10-11), 1091-1109 (2009).
2. H. Usta, A. Facchetti and T. J. Marks, Accounts Chem Res 44 (7), 501-510 (2011).
3. S.-O. Kim, T. K. An, J. Chen, I. Kang, S. H. Kang, D. S. Chung, C. E. Park, Y.-H. Kim and S.-K. Kwon, Adv Funct Mater 21 (9), 1616-1623 (2011).
4. T. K. An, S. H. Jang, S.-O. Kim, J. Jang, J. Hwang, H. Cha, Y. R. Noh, S. B. Yoon, Y. J. Yoon, L. H. Kim, D. S. Chung, S.-K. Kwon, Y.-H. Kim, S.-G. Lee and C. E. Park, Chemistry (Weinheim an der Bergstrasse, Germany) 19 (42), 14052-14060 (2013).

Various types of self-assembled monolayers (SAM) on insulators have been used to improve the performance of organic thin films transistors (OTFTs) [1]. In this work, a phenyl SAM (Ph-SAM) was applied to the AlO_x deposited by atomic layer deposition on thienothiophene derivatives (C10-DNTT) as the organic semiconductor material. Bottom Gate Top Contact OTFTs were fabricated using vacuum evaporated Cr as the gate electrode and vacuum evaporated Au as source/drain electrodes. Interface state densities have been characterized by current deep level transient spectroscopy (DLTS) with a bipolar rectangular weighting function in the unit of coulomb [2]. DLTS measurements were carried out under isothermal conditions at 300 K by applying the gate bias pulse for OTFTs with the source electrode shorted to the drain electrode. The interface state density at the emission activation energy of 0.8 eV was estimated to be 7.7x10¹¹ cm^-2eV^-1 for the OTFT with the Ph-SAM and was reduced to half of the interface state density for the OTFT without the SAM. This reduction of interface state density by using the SAM is promising to improve the performance of OTFTs. The interface state densities with shallower energy levels will be presented to confirm the effect of the Ph-SAM on the interface state distributions.
The authors acknowledge Nippon Kayaku Co., Ltd for supplying the DNTT materials. This work was financially supported by NEDO.
[1] K. Suemori et al., Appl. Phys. Lett. 91, 192112, 2007
[2] K. Nakamura et al., Materials Research Society Fall meeting, M5.80, 2013.

We report the growth of high-quality, pentacene-doped p-terphenyl single crystals for the realization of Microwave Amplification by Stimulated Emission of Radiation (MASER) in solid-state devices at room temperature. Zone-melting methods are used. The pentacene dopant concentration is determined using calibrated UV-Vis absorption measurements. Crystal quality and orientation are determined by x-ray diffraction (XRD) in conjunction with optical birefringence and mechanical cleavage. As grown ingots are trimmed, cleaved, oriented and polished to provide monocrystalline specimens suitable for photo-excited (pulsed) electron paramagnetic resonance (EPR) measurements. By modeling the observed EPR transients, the spin-lattice relaxation rates and lifetimes that control the viability and perform of these sorts of MASERs are determined.

Recently, polymer field-effect transistors (PFETs) have been motivating both academic and industrial researchers because of their continuous improvements in carrier mobility and stability. PFETs are promising for use in “plastics electronics” with unique advantages that include flexibility, transparency, and most importantly solution-processibility conducive to high throughput manufacturing methods such as roll-to-roll coating and inkjet printing. Understanding and controlling the factors influencing molecular assembly and charge transport of solution-processed PFETs is crucial for material design and device optimization toward industrialization. In this study, we investigated the hole transport and molecular assembly in the record hole mobility PFETs fabricated from the regioregular poly[4-(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b]dithiophen-2-yl)-alt-[1,2,5]thiadiazolo[3,4-c]pyridine], PCDTPT. The field-effect mobility can be significantly improved by introducing nano-grooves on the substrates and controlling the evaporation rate of the drop-cast solution. By aligning the polymer in macroscopic fiber bundles along the FET channels, hole mobility as high as 36 cm²/Vs has been achieved. High resolution atomic force microscopy and Grazing incidence wide-angle X-ray diffraction characterizations reveal the alignment of single polymer chain inside the fiber bundle. This alignment facilitates carrier transport along the conjugated polymer backbone with occasional hopping to neighboring chains. The energetic barrier for this hopping is found to be very small (~25 meV) by the temperature-dependent mobility measurement. Interestingly, the temperature-dependent mobility of these PFETs from 300 K to 80 K shows two regimes with different activation energies, which might arise from two different transport mechanisms.

We investigated the nanohybrid interlayers (Interlayers) in enhancing charge injections of both holes and electrons in ambipolar (poly(thienlylenevinylene-co-phthalimide)s functionalized at the imide nitrogen with 2-ethylhexyl chain (PTVPhI-Eh)) organic field-effect transistors (OFETs). The Interlayers are prepared by a facile method with using conjugated polymers (CPs) to wrap semiconducting carbon nanotubes by a mild centrifugation, without additional and expensive processes. In the contact-limited ambipolar OFETs, Interlayers lead to significantly lower contact resistance and increased mobilities for both carriers of holes and electrons. The resulting PTVPhI-Eh OFETs with poly(9,9-di-n-octylfluorene-alt-benzothiadiazole (F8BT) wrapped CNT interlayer (F8BT/CNT) show very balanced ambipolar transport properties with hole mobility of 0.3 cm²V^-1S^-1 that is enhanced by a factor of 3 and electron mobility of 0.22 cm²V^-1S^-1 which is improved by approximately 100 times, as compared with those devices without Interlayers. We also applied NI to PTVPhI-Eh devices using CP of poly(9,9-dioctylfluorene) (PFO) and observed device performance similar to that of OFETs with F8BT/CNT at similar CNT:polymer ratio, implying that the chirality of CNT is not important to Interlayers. Interestingly, very small amount of CNT (~0.06 mu;gmiddot;ml^-1) in the interlayers critically influences the charge injection behaviors, indicative of the dominant role of CNT. After a detailed examination of charge transport with low-temperature measurement, we found that such improvements are owing to the better charge injection at metal/OSC contact interface by using Interlayers. Finally, we demonstrate complementary inverters based on the ambipolar OFETs incorporating Interlayers without additional patterning.

Grain boundaries in organic semiconductor thin films act as bottlenecks to charge transport in organic field-effect transistors comprising polycrystalline active layers. Due to the negative impact on device performance, understanding how these boundaries influence charge transport is significant to the performance of these devices. Upon spin-coating, thin films of triethylsilylethynyl anthradithiophene (TES ADT) exhibit limited order, but subsequent exposure to 1,2-dichloroethane vapor induces growth of spherulites. Adjacent spherulites are separated by a mixture of low-angle (LA) and high-angle (HA) interspherulite boundaries (ISBs). Since the spherulitic growth rate can be significantly tuned by adjusting the underlying substrate&’s surface energy, we can pattern channels to direct the growth front of TES ADT and prescribe the formation of ISBs over macroscopic distances. This ability allows us to form exclusively LA (0±20°) and HA ISBs (90±20°). We perform gated four-point probe transistor measurements to quantify the resistance within spherulites and across these LA and HA ISBs as the devices are switched from “off” to “on” states. Previously, the cause of additional resistance at grain boundaries has been attributed to additional trap states induced by molecular disorder at the boundary. We find that for devices in the “on” state, the gate-independent resistances across ISBs remain quantitatively different from that within an individual spherulite. This result suggests that, even after all traps are filled, the angle of mismatch of the ISB continues to limit charge transport. We find that TES ADT displays thermally-activated charge transport behavior from 195K to 295K, both within spherulites and across interspherulite boundaries, with the activation energy nearly doubling across a HA ISB. However, this increase is much less than what has been previously found across grain boundaries in thermally-evaporated films, suggesting that the grain boundaries in solution-processed thin films are fundamentally less resistive than those in thermally-evaporated films.

Crystalline organic molecules often exhibit the ability to form multiple crystal structures depending on the processing conditions. Exploiting this polymorphism to optimize molecular orbital overlap between adjacent molecules within the unit lattice of conjugated polymers is an approach to enhance charge transport within the material. We have demonstrated the formation of tighter π-π stacking poly(3-hexylthiophene-2,5-diyl) polymorphs in films spin coated from ferrocene-containing solutions using grazing incident X-ray diffraction. As a result, we found that the addition of ferrocene to casting solutions yields thin-film transistors which exhibit significantly higher source-drain current and charge mobilities than neat polymer devices. Insights gleaned from ferrocene/poly(3-hexylthiophene) mixtures can serve as a template for selection and optimization of next generation small molecule/polymer systems possessing greater baseline charge mobilities. Ultimately, the development of such techniques to enhance the characteristics of organic transistors without imparting high costs or loss of advantageous properties will be a critical factor determining the future of organic components within the electronics market.

Anthracene-containing poly(p-phenylene-ethynylene)-alt-poly(p-phenylene-vinylene) (PPE-PPV) polymers can be used in many optoelectronic applications such as organic solar cells, organic field effect transistor and organic light emitting diodes.^{1, 2}
Here, AnE-PVstat polymer, equally equipped with linear octyl and branched 2-ethylhexyl side chains at the PPE and PPV parts, was synthesized according to a well established procedure which uses the Horner-Wadsworth-Emmons olefination reaction to polycondensate dialdehydes and bisphosphonate esters.³ In our pervious work, bulk heterojunction solar cells based on the polymer AnE-PVstat:[60]PCBM showed power-conversion efficiency of 3.03%. In this work, we present that the photovoltaic performance of AnE-PVstat was improved significantly to the level of 4.46% when active layer was prepared by solvent annealing method. Optical and charge transport properties were studied to elucidate device efficiency enhancement. Such a performance from a wide band gap polymer is rather appreciated for its application for organic solar cells in tandem architecture.
References
1. Usluer, O.; Kastner, C.; Abbas, M.; Ulbricht, C.; Cimrova, V.;Wild, A.; Birckner, E.; Tekin, N.;Sariciftci, N. S.; Hoppe, H.; Rathgeber, S.; Egbe, D. A. M. J. Polym. Sci., Part A: Polym.Chem. 2012, 50, 3425.
2. White, M.; Kaltenbrunner, M.; Glowacki, E.; Gutnichenko, K.; Kettlgruber, G.; Graz, I.;Aazou, S.; Ulbricht, C.; Egbe, D. A. M.; Miron, M. C.; Major, Z.;Scharber, M. C.; Sekitani, T.; Someya, T.; Bauer, S.;Sariciftci, N. S. Nature Photonics 2013, 7, 811.
3. Egbe, D. A. M.; Türk, S.; Rathgeber, S.; Kühnlenz, F.; Jadhav, R.; Wild, A.; Birckner, E.; Adam, G.; Pivrikas, A.; Cimrova, V.; Knör, G.; Sariciftci, N. S.; Hoppe, H. Macromolecules 2010, 43,1261.

Environmental effects, materials, and processing techniques are critical aspects challenging the commercialization of plastic solar cells. Different solvent systems have been studied to improve the morphology of plastic solar cells via solubility and evaporation effects. We show that good solar cell performance is achieved by using a non-halogenated solvent. This is done by creating more perfect crystals and forming nanophase donor/ acceptor separation under shear flow to make good electron and hole conducting pathways, thereby reducing carrier recombination. Therefore, the measured device efficiencies were similar to typical poly (3-hexylthiophene) (P3HT)/ phenyl-C₆₁-butyric acid methyl ester (PCBM) devices reported in the literature. Toluene is used here as the solvent which is less toxic and less costly than the typical halogenated solvents usually used. While P3HT is able to dissolve in toluene, PCBM is less soluble, resulting in poor solar cell performance. To overcome this both components were dissolved in a solvent mixture at a higher temperature, then the system was cooled to induce crystallization. To temper the crystallization the liquid was sheared resulting in long, single crystals of P3HT and phase separated PCBM. The liquid had a gel-like consistency allowing simple application onto substrates with a paintbrush resulting in devices of similar efficiency to those made with contemporary techniques. In summary, a paint-like liquid has been made which can be used to simply manufacture solar cells which has the morphology already present in liquid form. This is in contrast to presently used systems that form the morphology during application.

In our group, we are interested in studying new thin film synthesis using click chemistry. This bottom approach method has the advantage of achieving a sub-angstrom control of thickness, and can be easily tunable for a variety of potential application like thin film coatings, DNA sensors, and molecular electronics. Wire growth starts with self-assembly of aldehyde or amine terminate thiophenol molecules followed by alternate addition of terepthaldehyde and phenylenediamine molecules to form oligophenyleneimine (OPI) wires ranging from 0.6 - 7.3 nm in length. However, knowledge of the chain length distribution and surface coverage of OPI wires remains limited due to difficulty of characterizing monolayers instead of thick films, and is very pivotal to relate the structure and property for potential applications. By systematically capping the wires with a redox center and halogen, we were able to characterize the OPI wires molecule by molecule via Rutherford backscattering spectroscopy (RBS), Nuclear reaction analysis (NRA), X-ray photoelectron spectroscopy (XPS), Cyclic Voltammetry (CV), and Reflection-absorption infra-red spectroscopy (RAIRS). Our comprehensive results show that the wires can be grown at a reasonable surface coverage, and characterized step-wise for each imine condensation reaction. Furthermore, the methods used can be integrated to obtain similar results for a variety of wire architectures.

Exciton or energy transport in organic crystals is commonly described by a series of incoherent hoppings. However, this picture is no longer valid if the transport range is on the order of the exciton delocalization size. Recently, it is proposed that coherent mechanisms can enhance exciton transport in the nm length scale and may play a critical role in charge separation in organic photovoltaics. In this work, we use time-resolved photoemission spectroscopy to study coherent transport in various phthalocyanine (Pc) crystals. In ZnPc, we observe a transition from coherent to incoherent transport channel at around 350 fs after photoexcitation that is resulting from the decrease in delocalization size. The coherent transport range is around 2 - 3 nm prior localization. Interestingly, a distinct phonon mode is excited during the localization, which suggests that the electron-vibrational interaction is the main driving force to localize excitons and reduces its coherent size. By comparing the results from various Pc crystals, the effect of electron-vibrational coupling on the coherent transport range is investigated.

We developed high-efficiency green OLEDs using thermally activated delayed fluorescent (TADF) emitter. We used 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) as an emitter. For the electron transporter, we used a phenyl pyridine-based material named bis-4,6-(3,5-di-4-pyridylphenyl)-2-methylpyrimidine (B4PyMPM) with high triplet energy of 2.8 eV, electron mobility of 10^-4 cm² V^-1 s^-1 and high hole-blocking ability due to the deep ionization potential of 7.1 eV. By using a carrier- and exciton-confinement device structure, we realized a turn on voltage of 2.4 V at 1 cd cm^-2, a power efficiency (PE) of 70 lm W^-1 and high external quantum efficiency (EQE) over 23 %. This device also showed a very low driving voltage of 4.0 V, a PE of 49 lm W^-1 and high EQE of 20% at 1000 cd cm^-2. These are the highest PE among 4CzIPN-based TADF OLEDs so far.

We designed and synthesized a series of novel fluorine-substituted phenylpyridine-based electron transporters (ETLs) named 5&’-fluoro-3,5,3&’&’,5&’&’-tetra-3-pyridyl-[1,1&’;3&’,1&’&’]terphenyl (B3PyMFB) and 5&’-fluoro-3,5,3&’&’,5&’&’-tetra-4-pyridyl-[1,1&’;3&’,1&’&’]terphenyl (B4PyMFB) for blue phosphorescent organic light-emitting devices (OLEDs). Thermal properties of BPyMFB derivatives are greatly improved by fluorine-substitution compared with the corresponding non-fluorine-substituted ETL named 3,5,3,5-tetra-3-pyridyl-[1,1;3,1]terphenyl (B3PyPB). By using a variable angle spectroscopic ellipsometry (VASE), the orientation order parameter (S) was evaluated to be #8210;0.23 for B3PyMFB and #8210;0.21 for B3PyPB, respectively. Introduction of a fluorine atom into the core-skeleton gives almost same molecular orientation. We used B3PyMFB and B4PyMFB for iridium(III)bis[4,6-(difluorophenyl)pyridinato-N,C&’]picolinate (FIrpic)-based OLEDs. By using a combination of B3PyPB and B3PyMFB as an ETL, a highly efficient blue OLED with a power efficiency of 40 lm/W at 100 cd/m² was realized. This performance is higher than that of B3PyPB-based OLED most likely due to a better electron injection property of B3PyMFB.

Kelvin-probe force microscopy (KFM) is a powerful tool for local potential measurements with a nanometer-scale resolution, and has been widely used to characterize a variety of electronic materials and devices. For instance, KFM has been utilized for mapping surface potential of operating organic field-effect transistors (OFETs) to evaluate parameters such as contact resistance and charge trap density. In this study, we demonstrate a novel approach to measure local threshold voltage in OFET channel using frequency-modulation (FM) KFM. By obtaining two-dimensional (2D) map of the local threshold voltage in the organic thin film, we can obtain more detailed information on the carrier injection and channel formation.
Local threshold voltage was calculated from the relationship between the surface potential measured by FM-KFM and the gate voltage (V_G)[1,2]. For a p-type OFET channel, when V_G is negative with respect to the electrically grounded source and drain electrodes, carriers accumulated in the channel screen V_G, resulting in zero surface potential. On the other hand, when V_G is larger than a threshold voltage (V&’_TH), carriers in the channel are depleted and V_G is no longer screened, resulting in the increase of the surface potential with a slope of unity. V&’_TH is defined as the local threshold voltage. We obtained V&’_TH map by measuring the surface potential versus V_G curve at each pixel in the scan area and calculating V&’_TH as the threshold voltage between the two regimes.
We performed local threshold voltage mapping on a dinaphtho-thieno-thiophene (DNTT) OFET with the bottom-contact configuration. The obtained map showed that the local threshold voltage in the channel area was relatively small (near to zero), but it gradually became large (far from zero) with increasing distance from the electrodes, suggesting that the local threshold voltage was low at the area where carriers were easily injected. Moreover, it was confirmed by electrical measurement that the gate voltage at which the drain current began to flow through the channel coincided well with the V&’_TH measured in the channel area.
[1] E. M. Muller and J. A. Marohn, Adv. Mater. 17, 1410 (2005).
[2] M. Heike, Kotai Butsuri 42, 765 (2007).

We report optical properties and crystalline structures of thin-films of non-peripheral octahexylphthalocyanine (C6PcH₂), which was reported to exhibit carrier mobility of higher than 1 cm²V^-1s^-1 in time-of-flight method^[1] and has been utilized for a photovoltaic cells with the high power conversion efficiency of exceeding 4 %^[2]. By controlling temperatures of substrates and solutions during spin-coating process, diameters of domains in fabricated C6PcH₂ thin-films increased from lower than 1 mu;m to several ten mu;m, and uniform alignment of optic axes in the area with diameters of exceeding 1 mm is observed in the films fabricated at 150 °C. In optical absorption measurements of the films, dichroic ratios of about 5 were demonstrated at the wavelength of 740 nm corresponding to intra-molecular transition. Additionally, absorption intensity and shape of absorption spectrum drastically changed with angle of incident light respect to the substrate. From these results, π-stacked columner structure of C6PcH₂ suggested to be uniaxial-oriented with keeping tilt angles of molecules respect to substrates. In addition to the optical properties, surface profiles and X-ray diffraction patterns of the fabricated films also depend on process temperature. We discuss about the process temperature dependence of optical properties and crystalline structures of the films by taking the liquid crystallinity of C6PcH₂ into account.
Acknowledgement
This work was partly supported by Grant-in-Aid for JSPS Fellows.
[1] Y. Miyake et al., Appl. Phys. Express 4 (2011) 021604.
[2] Q. D. Dao et al., Appl. Phys. Lett. 101 (2012) 263301.

X-ray Photoemission Spectroscopy (XPS) is a powerful tool for determining chemical composition of materials. It allows not only to determine elemental composition of a given material, but also chemical states of the elements present in the sample. It is intrinsically a surface sensitive-technique. Although X-rays penetrate deep inside the sample, the photoelectron mean-free path is typically very short, which limits the signal acquired in the experiment to a surface layer of around 5 nm thickness. Angle-resolved experiments provide a means to measure changes in chemical composition of a sample with respect to depth, but do not give access to regions deeper than several nm.
Though, these deeper regions can be accessed by sputtering away outer layers of the sample with an ion beam. In XPS most depth profiling experiments have so far been performed with monoatomic ion beams operated at around 10-30 keV. These energetic ions introduce damage to the analyzed surface, which is particularly severe in case of organic materials. However, sputtering with large ionized clusters of atoms (containing up to several thousand of atoms) leads to very much reduced damage. Moreover, ionized clusters deposit most of their energy at the sample surface [1] and do not penetrate deep inside the analyzed material. In each sputtering cycle the destroyed portion of the sample is removed, revealing a fresh non-destructed surface for analysis. As a result XPS measurements combined with cluster ion sputtering yield information on depth distribution of sample components unaffected by ion damage. Literature reports [2-4] concerning organic materials and especially organic semiconductors studied by this combination of experimental techniques have so far been relatively scant.
Here I will present our results concerning thin films of organic semiconducting materials, mostly poly(3-alkylthiophenes). I will compare damage induced by massive argon ion clusters with that of monoatomic ions. I will also show how the composition of a thin polymer film can be determined accurately by means of this technique. AFM data will also be presented to reveal the influence of argon cluster beam on the topography and roughness of thin films. I will demonstrate how this combination of techniques may prove useful in studying organic electronic devices, including transistors, photovoltaics and sensors.
This work was partially supported by the Polish National Science Centre, project No. 2013/09/B/ST4/02951.
[1] Z. Postawa et. al, Anal. Chem. 75 (2003) 4402; Surf. Interface Anal. 43 (2011) 12
[2] M. Tanaka et al., Rapid Commun. Mass Spectrom. 2010; 24: 1405-1410
[3] N. Toyoda et al., Nucl. Instr. and Met. in Phys. Res. B 273 (2012) 11-14
[4] S. Ninomiya, Surf. Interface Anal. 2011, 43, 221-224

Carrier injection from metal electrodes to organic semiconductors is one of the major issues in organic field-effect transistors (FETs). Metal-organic interfaces are generally considered to be close to the Schottky limit, where a Schottky barrier height is determined by a difference between the Fermi level of a metal and the band edge of a semiconductor. For example, air-sensitive metals are widely used for electron injection to make injection barrier lower, and that interferes practical development of organic FETs. Therefore, it is required to develop a method to inject carriers employing stable metals. Here, we report symmetrical hole and electron injection to organic (2,5-bis(4-biphenylyl)bithiophene) thin-film transistors from gold electrodes by modifying their dielectric layer surface. Their carrier mobilities became either electrode&’s work-function sensitive or insensitive depending on the surface modification of the dielectric layer. Employing this method we have successfully demonstrated bright light emission in an ambipolar organic FET with gold electrodes. Detailed analysis on the injection barrier heights with temperature dependence measurements and photoelectron yield spectroscopy will be discussed in the presentation.

During the past decades, conjugated polymers as semiconductors have been extensively studied for organic electronics due to the advantages such as solution processability, the compatibility on flexible substrates and the possibility of roll-to-roll manufacturing. Recent design strategies such as a combination of electron donor (D) and electron acceptor (A) and substitution of alkyl functional group as side chain enabled polymers to have efficient intra-charge transport and to form a well-defined morphology. D-A conjugated polymers based on thienylenevinylene derivatives, in particular, have showed good performances in organic electronics because of its planarity and superior π-orbital overlap property. In this work, we synthesized donor-acceptor polymers with both vinyl group between thiophene units and alkyl functional group on backbone. The vinyl group is able to enhance a charge transport by extending π-conjugated length facilitating planarity of polymer backbone, and the functionalized alkyl chains control the morphology of polymer in solid state and improve solubility. As well as studies of chemical structure and material properties using ¹H-NMR, GPC, DSC, UV-vis spectroscopy and CV, analysis through XRD and AFM measurement were performed for a solid-state characterization. In addition, the polymer based thin film devices are also fabricated and characterized by simple solution process.

The solution processed organic solar cells are prepared by simple deposition techniques having a strong potential for the mass production of flexible and low cost devices.
The most common architecture is the bulk heterojunction solar cells, where a p-type donor semiconductor is blended with a n-type acceptor material. Efficiencies up to 10% where obtained by the development on novel donor materials combined with the morphology control of the active layer and the device engineering. In most of the cases, fullerene based derivatives were used as acceptor materials. Drawbacks of these fullerenes, such as weak absorption, difficult chemical tuning, high production costs, oriented the scientific research in finding alternatives for these acceptors.
Perylene diimide (PDI) derivatives are a class of robust and high light absorbing materials that can combine good electron mobility and suitable electronic levels to be used as acceptors to blend to donor polymers in bulk heterojunction solar cells. Despite such encouraging characteristics of PDIs, the photovoltaic performances of bulk heterojunction solar cells that use PDIs as acceptors are often far below expectations. One of the reasons limiting the efficiency in PDIs bulk heterojunction solar cells has been identified by the strong tendency of PDI to form aggregates. In a previous work we have shown that the insertion of naphthalene and acenaphtene in bay position is an effective strategy to decrease PDI aggregation and to improve the photovoltaic performances in P3HT:PDIs blend solar cells [1].
In this work we report the design and synthesis of a series of novel PDI molecules where bulky aromatic substituents has been linked in bay position as spirobifluorene and bithienyl-spirofluorene moieties. The characterization of these materials in solar cells with P3HT is presented and discussed. Through the use of several characterizations techniques, as ciclo-voltammetry, absorption and fluorescence spectroscopy, AFM and charge mobility measurements, an insight is provided about the relationship between the molecular design of these materials and their functionality as active components in bulk heterojunction solar cells.
[1]E. Kozma, D. Kotowski, S. Luzzati, M. Catellani, F. Bertini, A. Famulari, G. Raos, RSC Advances2013, 3, 9185-9188; E. Kozma, D. Kotowski, M. Catellani, S. Luzzati, A. Famulari, F. Bertini, Dyes and Pigments2013, 99(2), 329-338

Theoretical estimations of the maximal achievable open circuit voltage (V_OC) of organic solar cells become useless in many cases when the electrochemistry data are obtained for individual donor and acceptor components under different conditions. In the present work we have succeeded in performing the electrochemistry measurements for the composites of conjugated polymers with fullerene derivatives. This method allows one to estimate HOMO (D) and LUMO (A) values directly from a single measurement performed for a solid composite film resembling closely photoactive layer of a real solar cell. More than 30 different composite systems were investigated using cyclic voltammetry measurements and also evaluated in bulk heterojunction solar cells. An empirical equation was proposed to estimate achievable V_OC for various polymer/fullerene derivative composites. A correlation between the predicted and experimental V_OC values for the investigated systems is characterized by a Pearson linear correlation coefficient k=0.93. The obtained results proved that this method gives rather good estimation for the open circuit voltages achievable for many material combinations. We believe that the proposed experimental approach will be used extensively for evaluation of novel conjugated polymers designed for photovoltaic applications.

Organic electronic devices are attractive for a range of existing and emerging electronic applications. To date, most technological demonstrations employing organic transistors use them as switches and rely on their large signal response for pixel control or logic. However, there exists considerable application space which requires analog circuits, e.g. distributed signal conditioning in sensor arrays. Charge transport and trapping mechanisms differ significantly in organic transistors as compared to inorganic transistors, and as a result commonly used analogies of organic semiconductors to inorganic band transport theory can break down, especially in response to the small signal stimulus and at high frequencies required in some analog circuit applications. Therefore, a detailed investigation of organic transistor behavior at small signals is needed and is critical to developing design models for analog circuit applications.
In this study, we look at the small signal AC behavior of small molecule, single crystal transistors of rubrene and 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (diF TES ADT) to investigate "ideal" electronic behavior in organic transistors, including investigation of interface states at the dielectric/semiconductor surface using substrates treated with monolayers of octadecyltrichlorosilane (OTS). Impedance measurements allow us access to small signal, high frequency behavior and allow for more nuanced investigation of the small signal response and linearity of the transistor properties, trapped vs. mobile charge, and the determination of the low field mobility and threshold voltage. Using a transmission line model to fit the AC impedance of the transistor channel coupled with a parallel resistor-capacitor model of the contact impedance, we gain insight into charge transport within the channel while discriminating the effects due to contact impedance. We also evaluate classical small signal AC device analysis and modelling approaches to effectively describe the transistor gain-bandwidth. We compare mobility, threshold voltage, and characteristic transistor channel response determined by impedance measurements to values found through more widely employed DC characterization of transistors to verify the effectiveness of DC measurements to predict small signal performance limits. The AC analysis can be extended to determine mobility values in transistors with high trap concentrations or hysteresis where analysis can be difficult using only DC characterization.

Realizing an electrically driven organic solid laser is still a challenge though the optically pumped one has already been realized for many years. As we know that a laser consists nothing but a gain medium in a resonator, investigating the gain properties of the materials in the active layer is definitely a crucial factor besides improvement of the techniques in fabricating devices. Among numerous reports of amplified spontaneous emission (ASE) properties in organic single crystals, some of the materials showed dual gain narrowing peaks, but no one has given an exact explanation so far.^1,2 To invest the reason of the aforementioned phenomenon, we selected a newly synthesized material 2-(4-biphenyl)-5-[5-(4-biphenyl)-2-thienyl]furan (BPFT) as a substance, which showed dual gain narrowing peaks during ASE characterization. We also employed other two materials with similar molecular structures, 2,5-bis(4-biphenylyl) bithiophene (BP2T) and 2,5-bis(4-biphenylyl)bifuran (BP2F), which showed only one gain narrowing peak as comparison. Comparing both the absorption and fluorescence spectra of these three thiophene/furan/phenylene co-oligomers in the single crystals and deposited thin films, we determined that the dual gain narrowing peaks corresponding to the vibronic transitions. Combining the transient absorption spectra with regular absorption spectra, we finally determined that the number of gain narrowing peaks corresponding vibronic transitions from S₁₀ to S_0n is affected by both the re-absorption in the ground state and the excited state, and energy distribution of this gain behavior is determined by transition probability from S₁₀ to S_0n. The information obtained from the present experiments would be useful for realization of electric-current driven organic lasers.
References
¹ D. Fichou, S. Delysse and J. Nunzi, Adv. Mater. 9, 1178 (1997).
² P. A. Losio, C.Hunziker and P. Gunter, Appl. Phys. Lett. 90, 241103 (2007).

Periodic deposition of para-hexaphenyl (p6P) and α-sexithiophene (6T) molecules on a muscovite mica substrate by hot wall epitaxy results in multilayered, crystalline nanofibers. By varying the relative thicknesses of the constituent materials, the fluorescence spectrum can be tuned within the red, green, and blue spectral range [1, 2]. Illumination of the multilayered nanofiber sample with UV light (3.8 eV photon energy) directly excites fluorescence from the p6P layers, while the 6T layers emit light due to sensitization from the excited p6P [2]. Illumination with blue light (2.5 eV photon energy), which is below the optical gap of p6P, excites photoluminescence (PL) only from the 6T layers [3]. Here, we probe the energy transfer and exciton recombination characteristics by temperature dependent PL spectroscopy using either 3.8 eV or 2.5 eV photons for stimulating the p6P or 6T layers, respectively. While the p6P quantum yield decreases by an order of magnitude between 6 K and 293 K, the 6T monolayer emission exhibits much weaker temperature dependence. We tentatively attribute this to the fact that crystalline p6P layers are H aggregates while the ultrathin 6T layers have a radically different aggregation state (most presumably J aggregates). The ratio between emission intensity of the sensitized and directly excited 6T monolayer increases by around 40% when increasing the temperature from 6 K to 293 K due to enhanced energy transfer from p6P to the 6T monolayer. To further elucidate the exciton dynamics, we have used temperature-dependent, time-resolved PL spectroscopy that can provide quantitative data on the exciton diffusion and energy transfer processes. This improved understanding of the photophysics in organic heterocrystals can aid in the development of new nanomaterials with desired optical properties.
1 C. Simbrunner et al. ACS Nano 4, 6244 (2010)
2 C. Simbrunner et al. ACS Nano 6, 4629 (2012)
3 F. Quochi et al. Adv. Opt. Mat. 1, 117 (2013)

Measurements of photoluminescence yield over a wide range of excitation power in rubrene single crystals reveal a transition between a low-yield region and a region with a yield more than an order of magnitude larger. This transition occurs at an excitation density of 3 x 10²⁰ cm^-3 absorbed photons per second. This power dependence is predicted in case of an extremely efficient conversion between singlet and triplet excitons through fission and fusion. The high-yield region corresponds to the case when photoexcited singlet excitons undergo fission into triplet excitons that have a high probability of re-creating a singlet exciton by fusion until radiative recombination occurs. Triplet fusion starts contributing to rubrene's photoluminescence already at excitation intensities of 8 - 85 mW/cm² at room temperature depending on excitation wavelength and polarization, corresponding to triplet densities of ~ 2 x 10¹⁶ cm^-3. These results imply that in any experiments that rely on the detection of photoluminescence to reach conclusions about the efficiency of fission or fusion processes, it is essential to control the intensity of the excitation light to determine if an experiment is being performed in the low-yield or in the high-yield regime, i.e. with triplet exciton fusion contributing or not to the results.

The BHJ motif for OPV design is very creative, and can reflect substantial structural effects. The two important processes (photo preparation and charge collection) occur on time scales that can be very different, and that can respond to different aspects of the overall structure. We present some computational results that suggest varying opportunities and limitations for OPV design.

Optical spectroscopy is widely used to study the photophysical processes in organic photovoltaic (PV) material systems. In most of the spectroscopic techniques electrons and holes are generated simultaneously upon light excitation. In this work we demonstrated the use of charge modulation spectroscopy (CMS) to study localization of electrons or holes, without the interference from its counter charge, at organic donor-acceptor heterointerfaces. The CMS measurements were performed on a heterojunction field-effect transistor (FET) structure, allowing us to acquire spectra that separately represent the electronic states of electron or hole by changing the gate bias. Using this approach we studied two polymer systems, poly(N-alkyl diketopyrrolo-pyrrole dithienylthieno[3,2-b]thiophene) (PDPP-DTT), which exhibits record high hole mobility in FETs, and a thieno[3,4-b]thiophene-alt-benzodithiophene copolymercopolymer (PTB7), which yields high power conversion efficiency in a PV configuration. By comparing the frequency-dependent CMS spectra of the pure polymer, polymer/(6,6)-phenyl-C61-butyric acid methylester (PCBM) bilayer, and polymer/PCBM blend, we observe that while charge localization is greatly increased from bulk PTB7 to PTB7/PCBM interface, it remains almost unchanged for bulk PDPP-DTT and PDPP-DTT/PCBM interface. Such difference in the interface effect can be well correlated with the different spatial arrangement of PTB7 and PDPP-DTT molecules at the polymer/PCBM interface. Our study reveals a trade-off between charge transfer and charge trapping at the donor/acceptor interface in polymer bulk heterojunctions and suggests the need of creating an energy cascade at the interface to achieve a highly efficient charge separation process.

Modeling dynamical solute-solvent systems is a difficult task because a full quantum mechanical treatment of solute in solvent is prohibitively expensive. Continuum solvent models are a useful approach to this problem where the system is modeled as a solute embedded in a dielectric cavity. When the solute is electronically excited, the dielectric responds to various solute charge densities depending on the process of interest. The excitation calculation method can be classified as linear response when the dielectric responds to the solute transition density and state-specific when the dielectric relaxes with solute charge density for a specific excited state. Here, we describe efforts to describe these effects in excited state molecular dynamics using the random phase approximation and continuum solvent models, allowing quantum molecular dynamics simulations of excited state processes in solvents on realistic time scales.

We will use some of our recent results in the area of organic (bulk heterojunction and
bilayer) solar cells as well as organic field-effect transistors to highlight the complexity of
the electronic and optical processes taking place in these devices and the challenges they
pose for their reliable theoretical description. In particular, we will disccuss aspects related
to charge separation and to polarization effects.

The strong susceptibility to thermal lattice vibrations is one of the most fundamental differences of organic semiconductors compared to their inorganic counterparts. The resulting energetic disorder is believed to govern many of the remarkable electrical and optical properties in organic systems. However, dynamic disorder was until recently a topic solely for theoreticians as the underlying thermal vibrations were hard to access by experiment.
Last year we reported a novel technique based on electron diffraction that allows for direct probing of the most dominant lattice vibrational modes [1]. Since then we encountered strong thermal vibrations in every organic material we studied (10 up to date). This indicates that a large degree of dynamic disorder is a universal phenomenon in organic crystals. Interestingly, the dominant thermal vibrations seem to follow a particular pattern that is reproduced in all small molecules. Recently, we conducted electrical measurements that provide deeper experimental insight into the relationship between structural dynamics and charge transport. Our measurements show how the relative impact of static and dynamic disorder on mobility shifts with temperature. We found that at room temperature dynamic disorder often governs device performance in organic semiconductors. Our experiments provide a first insight of how lattice vibrations create different degrees of dynamic disorder in different crystal environments. Understanding these underlying mechanisms is crucial to control, and ultimately avoid dynamic disorder in order to develop the next generation of disorder free, high performance materials.
[1] S. Illig, A. S. Eggeman, A. Troisi, H. Sirringhaus and P. A. Midgley, Nat. Mater., 2013, 12, 1045-1049.

The temperature dependence of exciton diffusion length has been measured in a small molecule system as a function of processing conditions. Processing with a high boiling point additive leads to a reduction in the exciton diffusion length from 6.8 ± 0.4 nm to 4.9 ± 0.3 nm. Using Stern-Volmer analysis we find that additive processing also leads to emergence of excitonic trap states with density of 1.2 10¹⁸ cm^-3, which are responsible for reduction of exciton diffusion length. The temperature dependent measurements show that exciton diffusion is dominated by temperature activated hopping, while the contribution from downhill migration to the overall exciton diffusion length is significantly less than what has been previously observed in a polymer system.

The understanding of the charge transport mechanisms across ultra-thin organic semiconductors is one of the important keys for the engineering of emerging organic-based technologies. Recently, one of us have shown that the charge transport mechanisms across vertical heterojunctions based on physisorbed ultra-thin copper phthalocyanine (CuPc) films, can be precisely determined and controlled over a wide range of temperatures and electric fields [1]. In addition, for the Au/CuPc/Au heterojunctions, the observed macroscopic electrical characteristics are similar to what have been reported for chemisorbed molecular junctions [2]. For instance, the transition from thermally activated transport to tunneling is verified regardless of the nature of the molecule-contact bonding. Here we further employ this type of heterojunction to investigate the changes on charge transport mechanisms as the molecular layer gets thinner (from 40 nm to 4 nm). In order to avoid the interdiffusion of metallic atoms during the top electrode preparation, the heterojunction was prepared by combining standard microfabrication methods with the strain engineering of metallic nanomembranes [1,3]. Similar to what have been reported to chemisorbed molecular junctions, we observed that at high electric fields, the conduction across physisorbed CuPc-based heterojunctions is dominated by activated hopping for films thicker than 15 nm. Bellow this value tunneling takes place. At low electric fields, the activation transport can be observed down to 6.5 nm. We confirm that the conduction at high temperatures and low electric fields is mainly due to charge localization sites. By decreasing the temperature, a continuous transition from direct tunneling to field emission takes place by increasing the voltage bias for films bellow 15 nm. In this case, the charge transport across the localized sites is suppressed. In conclusion, the important macroscopic features commonly observed on electrical transport across chemisorbed molecular layers can also be manifested on physisorbed heterojunctions. This observation evidences that, depending on the application, the covalent bonding between electrode and molecule is, in some extent, not particularly essential.
[1] Bof Bufon, C.C. et al. J. Phys. Chem. C., 2014; [2] Yan, H. et al. PNAS, 2013; [3] Bof Bufon, C. C. et al. Nano Lett., 2011.

Ever growing complexity of nanomaterials requires robust and predictive multiscale approaches for their theoretical description. Combined with thorough experimental studies such approaches provide a reliable estimate of physical properties of nanostructured materials and enable a rational design of devices. From this perspective I will discuss first principle modeling of small-molecule bulk-heterojunction organic solar cells and push-pull chromophores for tunable-color organic light emitters. The emphasis is on electronic processes involving intra- and intermolecular energy or charge transfer driven by strong electron-phonon coupling inherent to pi-conjugated systems. Finally I will describe how precise manipulation and control of organic-organic interfaces in a photovoltaic device can increase its power conversion efficiency by 2-5 times in a model bilayer system. Applications of these design principles to practical architectures like bulk heterojunction devices lead to an enhancement in power conversion efficiency from 4.0% to 7.0%. These interface manipulation strategies are universally applicable to any donor-acceptor interface, making them both fundamentally interesting and technologically important for achieving high efficiency organic electronic devices.

Organic solar cells are the subject of an extensive research effort due to their potential application as cheap, light-weight and flexible energy sources. An important step towards the realization of commercialization is attainment of higher efficiencies and improvements of the scale-up process. Both objectives are possible only through a more complete understanding of factors limiting the device performance. In this talk, I will discuss the relationship between fill factor (FF) and hole and electron mobilities across a range of molecular donor material systems using phenyl-C71-butyric acid methyl ester (PC71BM) as the electron acceptor. The relationship between mobilities measured in neat films and blend films is also explored thereby illustrating the utility of neat film mobility measurements. Three classes of materials were studied: diketopyrrolopyrrole (DPP), oligothiophene and fluorobenzothiadiazole (FBT) based materials, which represent the most successful and well-studied classes of solution processable molecular donors used in BHJ solar cells. This diverse set of materials has yielded a large range in solar cell device performance with PCEs from 1% to 8%, FFs from 30% up to 70%, and hole and electron mobilities that vary over three orders of magnitude. Thus, this set of materials is ideal for exploring systematic trends and relationships that govern solar cell device performance.

Carrier multiplication is a possible mechanism for increasing solar cell efficiency beyond the Shockley-Queisser limit. Within organic systems, singlet fission (SF) is the primary mechanism proposed for implementation of carrier multiplication. Through SF, a singlet exciton can be split into two triplet excitons, which can then be split to form charge carriers. The result is quantum efficiency exceeding 100%. However, applications have been limited by the small number of materials that exhibit efficient SF. Additionally, existing materials that exhibit efficient SF are based on intermolecular interactions, which require long range order and good nearest neighbor coupling.
Here we propose and validate the first modular framework for designed intramolecular SF-capable materials. This new family of materials is based on strong-donor-strong-acceptor interactions in molecular and copolymer systems. The design is grounded in the charge-transfer mediated singlet fission mechanism that originated in studies of molecular singlet fission in organic crystals. Using this design framework, we synthesize and characterize the first materials in this new class. We use transient absorption and pulse radiolysis to confirm that all materials show significant SF. We quantify the SF yield to show that the best material displays a 170% triplet quantum yield. We then quantify and discuss the parasitic electronic processes that compete with SF. The design principles resulting from this study are widely applicable to a broad family of similar materials, where each moiety can be chosen to tailor the electronic, optical, and physical properties of the resulting polymer, thus allowing for the carrier mobility, bandgap, and solubility to be tuned.

Herein, we synthesized diphenyl bis[1]benzothieno[2,3-d:2prime;,3prime;-dprime;]naphtho[2,3-b:6,7-bprime;]dithiophene (DPh-BBTNDT) and examined organic field-effect transistor (OFET) characteristics by using its vapor-deposited thin film. The OFET characteristics were investigated using the bottom gate, top contact device structure on Si/SiO₂ substrates. Although the mobility largely depended on temperature of substrates (T_subs) during the deposition of the thin film, the devices showed excellent OFET characteristics with the mobility higher than 1 cm² / Vs at T_sub = 200 °C. In particular, the best mobility was up to 7.0 cm² / Vs on the ODTS-treated substrate, which is comparable to the highest mobility so far reported for small-molecule-based OFETs with multigrain thin films as the active semiconducting channel. In fact, the results of out-of-plane and in-plane X-ray diffraction analyses for DPh-BBTNDT in the thin film state indicated that the molecules tend to stand perpendicular to the substrates and take two-dimentionally interactive herringbone packing structure in the solid state, which rationalize the higher carrier mobility in the DPh-BBTNDT-based OFETs. Furthermore, DPh-BBTNDT ensured thermal stability in thin film state on the substrate, and in fact the OFET devices resisted thermal treatments up to 300 °C for 30 min under air. The morphology of the films will be also reported.

Low work-function electrodes are typically needed to inject electrons into the LUMO level of an organic semiconductor. However, this implies the use of thin metallic interlayers as for instance calcium or lithium fluoride, which are chemically reactive and easily oxidize in the presence of ambient oxygen and water. Recently, insulating polymers containing simple aliphatic amine groups such as the highly branched polyethylenimine (PEI) have been shown to be universal surface modifiers that allow the fabrication of air-stable low-work function electrodes for efficient electron injection. The presence of interfacial dipoles at the PEI/electrode interface has been cited many times as the reason for the enhanced device performances. However, the real mechanism responsible for such a performance appears to be more complex.
Here, we show that the vapor from the PEI sample contains N-based impurity molecules with a strong reducing character and that this is the prime mechanism behind making the electron injection efficient. A chemical reaction occurs at the PEI-vapor/organic semiconductor or at the PEI-vapor/electrode interfaces. In some cases we actually observe that the volatile molecular reducing agent, from the PEI sample, migrates throughout the entire bulk of the organic material. Hence, both the energetics at the interface and the electrical conductivity of the organic (semi)conductor bulk are changed. This would explain the performance enhancement in (opto)electronic devices upon inclusion of the ultrathin PEI layer.

As the upward trend in organic photovoltaic efficiency bolsters the viability their commercial manufacturing, it is important to consider the effect of large-scale fabrication techniques on morphology. To elucidate the morphology evolution of a high-efficiency, blade-coated organic photovoltaic (OPV) active layer containing the low band-gap polymer poly[(4,8-bis[5-(2-ethylhexyl)thiophene-2-yl]benzo[1,2-b:4,5-b']dithiophene) -2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiophene))-2,6-diyl] (PBDTTT-C-T) and PC₇₁BM. In-situ X-ray scattering experiments enable film characterization in real time and the effect of a processing additive, 1,8 diiodooctane (DIO) on active layer morphology. Small-angle X-ray scattering and energy-filtered TEM of DIO containing films exhibit smaller domain size, whereas grazing-incidence X-ray diffraction (GIXD) reveals increased order of the alkyl chain stacking. The solidification behavior of this active layer differs dramatically from that of poly(3-hexylthiophene) and small molecule donor containing systems, exposing an unexpected diversity in solidification routes for high-efficiency polymer-fullerene OPV processing.

We will present a discussion on how microstructural changes affect the electronic landscape of organic solar cells, including charge generation, charge recombination and charge transfer states. We will show, for instance, that the evolution of well-defined interfaces between intermixed and relatively phase-pure domains of the donor and acceptor is required for spatial separation of the electron and hole. This structural characteristic largely controls the yield of free charges. Moreover, we should that the intensity of CT absorption is directly correlated with the presence of a molecularly intermixed donor:acceptor phase and discuss what structural features determine the CT energy.

To sustain the rapid improvement in the electronic properties of new organic semicondutors, it is important to understand and ultimately engineer molecular connectivity over length-scales comparable to the transport dimension. Here we use high-resolution transmission electron microscopy (HRTEM) to examine a variety of well-preforming conjugated polymers and solution-processed small-molecules to determine size and shape of crystalline regions along with the special relationships that develop between crystals through their mutual interactions. These relationships are markers of the self-assembly processes and reflect both the molecular structure and processing conditions. Two distinct motifs are observed. First, nano-scale crystallites may locally align on scales approaching a micron even in well-performing BHJ layers (>7% PCE) and transistor materials. Second, neighboring crystallites often adopt specific epitaxial relationships. These relationships are expected to be important for both determining the morphology and may benefit the electronic properties. We demonstrate epitaxy in several well-preforming polymer materials, suggesting crossed chains at the interface between thin lamellae may couple electronically and create efficient quasi-3D charge transport pathways which can reduce the severity of grain boundaries and defects in limiting transport. The results suggest that long-range order and epitaxy may exist in a wide variety of well-performing materials and that intentionally engineering these aspects into new materials may lead to substantial advances in performance and control of the self-assembly process.

The use of small volumes of a high boiling point liquid as a ‘solvent additive&’ is a deposition processing method that has been implemented in most high/record performing polymer:fullerene based bulk heterojunction (BHJ) solar cell devices. Recently, solvent additive processing has been employed in a solution processable small molecule (SPSM) BHJ system, p-DTS(PTTh₂)₂:PC₇₁BM, where when a small amount, 0.25 v/v%, of the solvent additive 1,8-diiodooctane (DIO) was added to the casting solution, several key device metrics increased leading to a high power conversion efficiency (PCE) of 7%. X-ray diffraction experiments show that the amount of additive added to the casting solution to make p-DTS(PTTh₂)₂:PC₇₁BM thin films has several effects on the structure at multiple length scales: e.g. the number and orientation of p-DTS(PTTh₂)₂ crystallites, different π-π stacking distances, and the nano-scale domain size. Additionally, we show real space micrographs by a transmission electron microscopy (TEM) technique that enhanced the contrast between p-DTS(PTTh₂)₂ and PC₇₁BM that further verify the effect of increasing domain size as the additive concentration increases. Tomographic reconstruction of the TEM micrographs provides a 3D representation of the BHJ structure and shows how domain size and tortuosity in all dimensions change due to solvent additive processing where the main finding is that the nano-structures of p-DTS(PTTh₂)₂ have enhanced connectivity when 0.25 v/v% DIO is used. Finally, we show evidence of solvent additive retention in p-DTS(PTTh₂)₂:PC₇₁BM films when 1 v/v% DIO is used (but absent for 0.25%). This finding, in conjunction with the appearance of two populations of π-π stacking distances when 1 v/v/% DIO is used, may lead to the identification of one of the specific points of interaction of DIO with p-DTS(PTTh₂)₂.

Design rules to impart high carrier mobility to polymeric semiconductors have often been built under the assumption that a high degree of order and crystallinity is needed. Recently however, record-breaking semiconducting polymers have challenged this paradigm by exhibiting a high mobility and a surprisingly low degree of order. Indeed, charge transport in semicrystalline polymers depends on the complex interplay of different phases and their connectivity and cannot be simply related to overall crystallinity. In this talk, I will analyze the effect of the existence of multiple phases (crystalline and amorphous) on charge transport. The connectivity of the phases is also fundamental in ensuring high mobility. The scale-dependent nature of transport (fast along a chain and slow from chain to chain) plays a particularly important role in aggregate connectivity. Modeling charge transport in the disordered regions allows us to extract a proximity criterion between aggregates to ensure charge percolation through the aggregates. Experimental results on model materials clarify the role of local order and connectivity. By taking into account the multi-phase and scale-dependent nature of charge transport in polymers, we gain a fundamental understanding of why seemingly disordered polymers exhibit high mobility and we can extract general design rules for high-performance polymers.

In this presentation we will review recent advances in the design, synthesis, characterization and application of organic semiconducting materials and dopants. We will highlight the utility of C-H functionalization for the development of materials for organic electronics and opto-electronics.

The ability to store information is essential for many electronic applications, and polymers offer a unique platform to create non-volatile memory devices that are low-cost, lightweight, and amenable to large-area solution based deposition. One especially promising class of all-organic memory is resistive switches based on a phase separated blend of a ferroelectric polymer and a semiconducting polymer. As with most organic electronics, the thin film morphology of the active layer is critically linked to charge transport and overall device performance, but it is very difficult to predict a priori which materials will produce an optimal morphology. Phase separation of polymers frequently leads to large-scale domains (many microns) whereas for memory devices, phase separation on the 100 nm length scale is desirable. We sought to improve this morphology by tuning polymer-polymer interactions by alteration of the side chain structure of the semiconducting polymer, while maintaining the charge conduction properties of a polythiophene backbone. When blended with the typically used ferroelectric polymer, poly[(vinylidenefluoride-co-trifluoroethylene] (PVDF-TrFE), this semiconducting polymer, referred to as PEPT, demonstrates greatly reduced domain sizes, close to the 100 nm length scale, and much smoother films compared to previously studied polythiophenes. In addition to these significant improvements, this system shows semiconductor domains that are well defined with sizes that are easily tunable with PEPT content. Furthermore, PEPT is more crystalline than previously studied polythiophenes, and this opens opportunities to understand relationships between crystallinity and crystallite orientation and charge transport in ferroelectric-semiconductor blends. Relatively little has been done to understand the internal morphology of semiconductor-ferroelectric polymer blends and how this compares to the film surface. A combination of microscopy and X-ray based methods reveals that there may be subtle differences between surface and internal morphology, which has additional implications on design rules that are currently not well understood. These results demonstrate the importance of detailed structural characterization for ferroelectric-semiconductor polymer blends and reveal that side chain modification can significantly alter domain sizes and be used as a handle to optimize thin film morphology for organic memory applications.

Molecular n-dopants dope conjugated polymers either by reduction of the host material by direct electron transfer or by decomposition of a precursor to an intermediate that is capable of reducing the host. Two examples of molecular n-dopants are the dimethylbenzimidazolium salts (DMBI-X), which release unidentified reactive intermediates during a thermal deposition process, and the 1,3-dimethyl-2,3-dihydro-1H-benzimidazoles (DMBI-H) which dope by hydride transfer. In either case, additional carriers result from the same benzimidazolium cation/host radical anion pair, DMBI+/Abull;-, reaction product. For DMBI-I dopants, strong n-doping has only been observed for vacuum deposited films, and for DMBI-H dopants, the necessary hydride transfer step will limit the scope of dopable host materials. Here we report on the synthesis, characterization, and doping mechanism of benzimidazoline-radical dimers ((DMBI)2) and their use to form the doped state DMBI+/Abull;- in conjugated polymers. The distinct properties of the (DMBI)2 dopants relative to the DMBI-H dopants is evident from the reaction rate, polaron intensities, Fermi level shifts, and conductivities achieved using (2-Cyc-DMBI)2 and 2-Cyc-DMBI-H. The distinct reactions by which the dopants function results in greater variability of the doping effect for the DMBI-H compound than for the dimer in the hosts explored in this study. The present work demonstrates the importance of mechanistic understanding for achieving effective doping, and will advance the field by providing clear methodology for the preparation and utilization of potent (DMBI)2 dopants.

Organic electronics have been considered a leading candidate to make transparent and flexible electronics at a low cost. We have previously shown that the solution shearing method is a process that improves electrical performance for a range of OSCs, and the method is compatible with roll to roll industrial processing. This method can also tune the polymorph formation in OSCs, enabling high performance transistors without changing the OSC chemical structure. However, it is difficult to study the morphological and polymorph formations that enable high OTFT performance in situ. Not only does the thin film crystallize at a fast time scale, the evaporation front, where the crystal grows from the solution, is very small. The entire evaporation front can be less than 200 microns. Thus, the solution evolves into a crystallized thin film within seconds, and within an area less than 0.2 mm wide.
We use an X-ray ‘microbeam&’ at the Cornell High Energy Synchrotron Source, with a beam width of < 20 microns, in conjunction with a high speed detector to resolve and follow crystallization from solution of the OSC during solution shearing. We have collected up to 100 frames per second X-ray images, and are able to create grazing incidence x-ray diffraction movies to easily see how crystallization occurs in the solution shearing system in real time. We also use an optical microscope trained at the evaporation front, which we can use to collect optical videos of the evaporation front at up to 10,000 frames per second. Being able to simultaneously study kinetic crystallization using both optical and X-ray movies helps us understand how different processing conditions result in various polymorphs. We study the model OSC 6,13-bis(triisopropyl)-silylethynyl pentacene (TIPS-pentacene) and show that confinement of the growing thin film plays a key role in forming metastable polymorphs, and that the film formation proceeds downwards from the air-solution interface. We generate metastable crystal polymorphs through other solution processing conditions as well. This is the first time such a fast rate of data collection has been utilized for grazing incidence X-ray diffraction.

Organic photovoltaic device performance has improved rapidly since their invention^[1], with some new systems able to generate free charges with a quantum efficiency (QE) of even greater than 80%^[2]. This is unexpected, as organic materials have a low electrical permittivity^[3], and many devices exhibit losses due to charge recombination occuring on timescales of the order nanoseconds^[4]. Some groups have suggested that the nature of the charge transfer state, formed by the dissociation of a bound exciton at a donor-acceptor heterojunction, could explain the observed high QEs. It is proposed that, for small length and timescales, high-energy `hot' charge transfer states (HCTs) can be accessed, which delocalise the electron-hole pair, reducing Coulombic attraction between them and facilitating fast, efficient separation^[5].
We use Kinetic Monte Carlo (KMC) methods to explore the effects of HCTs on the free-charge generation for donor:acceptor (all-polymer) and mixed:aggregate (polymer:fullerene) devices^[6]. Increased fullerene aggregation is shown to increase the free-charge generation and short-circuit device current due to the production of HCTs at the mixed:aggregate interface. However, in order to observe the high QE reported by experiment, unphysical kinetics and HCT delocalisation (r) must be assumed. Efficient (> 50%) free-charge generation can be achieved when r ~ 10nm, at which point the Coulomb binding energy approaches k_BT and separation is energetically favoured. Such delocalisation is larger than the r ~ 4nm observed experimentally^[7]. At the literature values of r, it is shown that high QE is achievable for low free electron mobility mu; ~ 5x10^-6 cm² V^-1s^-1, which is unlikely to occur due to the high mobility measured in PC₆₀BM aggregates (mu; ~ 1x10-3 cm² V^-1s^{-1 [8]}).
Our data suggests that HCTs are unlikely to be the sole precursor to efficient free-charge generation observed in some polymer:fullerene OPVs. It seems that, while HCTs are indeed responsible for the fast (~ 100fs) separation observed in spectroscopy, regional variation in energy levels close to the heterojunction is more likely to increase the observed QE. As such, this work predicts that cascaded energy heterojunctions play a more significant role in OPV operation, especially as KMC simulations have been able to use experimentally verified calibration parameters to predict efficient free-charge generation^[9,10].
[1] G. Yu et al, Science, 1995, 270, 1789
[2] J. A. Love et al, Adv. Funct. Mater., 2013, 23, 5019
[3] S. F. Alvarado et al, Phys. Rev. Lett., 1998, 81, 1082
[4] I. A. Howard et al, J. Am. Chem. Soc., 2010, 132, 14866
[5] A. A. Bakulin et al, Science, 2012, 335, 1340
[6] M. L. Jones et al, Phys. Chem. Chem. Phys., 2014, In prep
[7] S. Gélinas et al, Science, 2014, 343, 512
[8] C. Waldauf et al, Adv. Mater., 2003, 15, 2084
[9] C. Groves, Energy Environ. Sci., 2013, 6, 1546
[10] T. M. Burke et al, Adv. Mater., 2014, 26, 1923

Fully conjugated block copolymers composed of donor and acceptor blocks can provide mesoscale and interfacial morphology control, leading to tunability of charge and energy transfer processes at donor/acceptor interfaces. Incorporating the donor-acceptor interface within the molecular structure at the block copolymer junction enables exquisite control of electronic coupling beyond systems relying on non-bonded interactions. We have recently shown that poly(3-hexylthiophene-2,5diyl)-block- poly-((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(thiophen-5-yl)-2,1,3-benzothiadiazole]-2&’,2”-diyl) block copolymer-based devices demonstrate efficient photoconversion well beyond devices composed of homopolymer blends. The 3% block copolymer device efficiencies are achieved without the use of a fullerene acceptor. Resonant soft X-ray scattering and energy-filtered transmission electron microscopy experiments corroborate that the efficient performance of block copolymer solar cells is due to self-assembly into mesoscale lamellar morphologies. Furthermore, we can also perturb the chemical structure to examine the role of the block copolymer chemistry on charge and energy transfer. Through a combination of device characterization and ultrafast spectroscopy, we find that small perturbations, for example in the highest occupied molecular orbital of the acceptor, result in large changes in charge photogeneration efficiency. Quantum calculations suggest that control of the chemistry can lead to charge transfer states near 1.8 V for P3HT-based block copolymers, resulting in experimentally measured open-circuit voltages beyond 1.2 V.

While conducting polymers have found wide spread use as electrode materials for optoelectronic devices, their electrochemical properties have led to numerous applications including electrochromic windows, mechanical actuators, and biosensors. Specifically, PEDOT, poly(3,4-ethylenedioxythiophene), doped with the polyanion PSS, poly(styrenesulfonate) is often employed due to its high conductivity, low impedance, and stability in aqueous environments. Many fields, including bioelectronics and energy storage, require materials that allow both efficient ionic and electronic transport leading to demanding requirements from a morphological perspective. In this work we combine electrochromic moving front experiments to quantify ion transport, conductivity measurements to determine electronic properties, and a suite of scattering and optical probes to relate microstructure/morphology to transport. Owing to its high disorder, low scattering contrast, and difficulty in studying the complexed homopolymers, we turn to resonant soft X-ray scattering techniques to deduce domain evolution for a series of PEDOT:PSS formulations. We show that structural parameters simultaneously describe the increased electronic conductivity and decreasing ionic conductivity when a co-solvent, ethylene glycol, is added to the conducting polymer dispersion. These studies represent a significant step towards control of multi-modal (ionic and electronic) transport and rational design of organic mixed conductors.

Microstructure of an organic semiconductor is strongly affected by surface characteristics of underlying substrates. Controlling this interface has been an important issue in organic electronics in particular to improve the performance of organic thin film transistors (OTFTs). In this talk I will introduce a new approach that utilizes vacuum-deposited organic heterointerface to improve the crystallinity and to control the morphology of organic thin films. m-Bis(triphenylsilyl)benzene (TSB3), the insulating soft small molecule with a moderate glass transition temperature, was used as the interfacial layer, and pentacene was deposited subsequently. The deposition of these two materials without breaking a vacuum gave rise to an extraordinary morphology with indistinctive grain boundaries and a myriad of nanometer-sized pores. We found that these peculiar structures are formed by different molecular interactions among the organic layers and the substrate surface. The pentacene film with TSB3 exhibited high field-effect mobility up to 6.3 cm² V^-1 s^-1, and the pore-rich structure much improved the sensitivity of organic-transistor-based chemical sensors. These results elucidate that our approach may find use in the fabrication of high-performance organic electronics applications.

Charge transport in conjugated polymers has a complex dependence on film morphology. Aligning the polymer chains in the plane of the film simplifies the morphology of the system allowing for insight into the morphological dependence of charge transport in these materials. Alignment also provides an opportunity to maximize charge carrier mobility where transport along the polymer backbone is typically greatest compared to other transport directions. Here, we demonstrate the effective alignment of the liquid crystalline polymer semiconductor poly(2,5-bis(3-tetradecylthiophen-2yl)thieno(3,2-b)thiophene) (pBTTT-C14) using large applied strain while on PDMS and held above its liquid crystal phase transition temperature. This approach represents a novel approach to align liquid crystal polymers. The morphology of the resulting film and charge transport characteristics are analyzed in detail including AFM, optical spectroscopy and X-ray scattering. The level of applied strain is shown to vary the level of in-plane backbone alignment. In addition, the strain-aligned films do not show the characteristic ribbon-phase commonly observed in highly aligned pBTTT films produced using other methods such as flow coating, providing a contrasting microstructure between aligned films. The field effect mobility is measured in these materials showing that intermediate strains (below strains of 50%) improves mobility in the alignment direction and that addition strain alignment results in a reduction in mobility as compared to a similar cast isotropic film. Combining the experimental results along with contrasting aligned film microstructures provides new insight into the physical origin of charge transport barriers in aligned pBTTT-C14 films.

Pi-conjugated semiconducting polymers are actively under development for use in organic light-emitting diodes, thin-film transistors, and solar cells. However, the understanding of their performance in many applications is limited because of the difficulty in establishing detailed structure-property relationships, which arises from the disconnect in our understanding of microstructure development. The lack of a unified understanding in microstructure formation, in turn, partly comes from our inability to precisely control defects in pi-conjugated semiconducting polymers. In this talk, we build upon our group&’s ability to perform controlled polymerizations for the synthesis of semiconducting polymers, specifically poly(3-hexylthiophene) (P3HT), and precisely alter defects within the single polymer chain. Using polymer nanowires as a model system, results will be shown that aims to understand how defects affect polymer microstructure.

Solution-processed organic field-effect transistors (OFETs) have been extensively explored due to the possibility to prepare low cost, printed, and hopefully disposable, electronic devices.^1,2 Among all conjugated structures investigated, many thiophene-diketopyrrolopyrrole (TDPP)-based copolymers have exhibited promising optical and electronic features with, in some cases, high carrier mobilities up to 12 cm²/Vmiddot;s.^3,4 However, most of those polymeric materials are produced by Stille coupling which requires additional synthetic steps for the organotin monomer and forms stannylated by-products. For a large-scale production of such DPP-based polymers, the utilization of the new direct (hetero)arylation polymerization (DHAP) could become a highly valuable tool. Unfortunately, reports points out that DHAP has one potential drawback which seems related to the lack of selectivity when different C-H bonds are present within the monomeric compounds.⁵ The TDPP moiety bears 3 reactive protons on each thiophene unit indicating that this unit could possibly leads to branched or cross-linked polymers. To overcome this issue, β-protected TDPPs were synthetized and copolymerized with protected and unprotected TDPP analogues in order to shed light on the selectivity and reactivity of this system. A high molecular weight polymer of 45 kDa was achieved with remarkably good yield of 84% from an unprotected TDPP. To the best of our knowledge, it is the first well-defined material synthetized by DHAP with monomers bearing multiple reactive C-H bonds. An impressive hole mobility, up to 1.17 cm²/Vmiddot;s, and low threshold voltage near 0 V was accomplished from a BGBC transistor configuration. A near ideal transfer curve was found, from the quasi-linear slope, implying a high quality semiconductor. These results suggest that couplings involving (hetero)aromatic units in DHAP could be more selective than expected and we are currently expanding our methodology to several key monomers used in the synthesis of conjugated polymers for organic electronics.
¹ Arias, A. C.; MacKenzie, J. D.; McCulloch, I.; Rivnay, J.; Salleo, A. Chem. Rev.2010, 110, 3.
² Sirringhaus, H. Adv. Mater.2014, 26, 1319.
³ Li, J.; Zhao, Y.; Tan, H. S.; Guo, Y.; Di, C. A.; Yu, G.; Liu, Y.; Lin, M.; Lim, S. H.; Zhou, Y.; Su, H.; Ong, B. S. Sci. Rep.2012, 2, 754.
⁴ Kang, I.; Yun, H. J.; Chung, D. S.; Kwon, S. K.; Kim, Y. H. J. Am. Chem. Soc.2013, 135, 14896.
⁵ Fujinami, Y. K., J.; Lu, W.; Hayashi, H.; Kanbara, T. ACS Macro Lett.2012, 1, 67.

My research group focuses on the chemistry of selenophene and tellurophene heterocycles. We have recently introduced selenophene-thiophene block copolymers and discovered that these rod-rod copolymers undergo a significant amount of phase separation. This is surprising given the chemical similarities between repeat units, however we have uncovered several properties, including crystal packing, that differ in these polyheterocycles. The polymers can be used to organize spherical CdSe nanocrystals into aligned hierarchical structures, and when combined with a fullerene derivative, they function as the light-harvesting component in a solar cell that has a greater thermal stability than cells composed of rod-like homo polymers. We have also learned how to synthesize polymers and delocalized molecules based on tellurophene, and identified several unexpected properties in these materials including reversible binding of small molecules and chemochromic behavior. More recently we have been focusing on so-called ‘n-type&’ organic materials. Here we focus on controlling their macromolecular structure, phase behavior, and we are applying them for energy storage applications including batteries and supercapacitors.

Semi-conducting polymers are promising for the development of low-cost flexible optoelectronic devices. These technologies are not yet mature and several limitations emerge. Synthetic routes rely on complex protocols using costly and hardly removable catalysts. Residual metal traces affect performances, involving numerous purification steps. It is necessary to develop versatile, economically viable and “green” approaches for large-scale syntheses.
In this contribution, we develop new synthetic tools for efficient polymers, by taking care of purity; variety of targeted structures and versatility of synthetic processes. Original polymers with aryl-vinyl repeating units are targeted with the possibility to tune (minimize) the energy gap, photo/electro luminescence properties. Original alternated squaraine-based π-conjugated polymers have been successfully obtained through a metal-free condensation reaction. They exhibit interesting optical properties, such as a strong absorption in red and near infra-red and good emitting properties. Finally, this original “green” synthetic route has been compared with more classical technique (Suzuki or Stille coupling) for the synthesis of the same model polymers. First results show a strong interest for this new synthetic approach.

Naphthalene diimide based donor-acceptor polymers are a promising class of electron accepting material that have demonstrated high n-type field effect mobility as well as promising performance as the electron accepting polymer in all polymer organic cells.^[1-2] Whilst there has been significant effort upon investigating the role of the co-monomer on polymer performance, there have been few reports on the role of the solubilising sidechain on polymer performance. Here we report simple synthetic routes to a variety of long chain, branched alkyl amines and report their incorporation into naphthalene diimide monomers. Polymerisation of the resulting monomers with bithiophene, as well as a range of donor monomers resulted in high molecular weight material. We report the influence of the sidechain length and branching point on the thin film morphology and thermal behaviour by a variety of techniques. We demonstrate differences of over one order of magnitude in field effect transistor mobility relating to the position of the branched point. In addition we also report the influence of molecular weight on transistor device performance. Differences in solar cell behaviour will also be discussed.
^[1] H. Yan et al., Nature, 2009, 457, 679.
^[2] H. Huang et al., Adv. Funct. Mater., 2014, 24, 2782.

Conjugated polymers posses many exceptional electronic, optical and thermal properties and thus are well suited for organic semiconducting applications, such as photovoltaic cells, thin film transistors, and light emitting diodes. However, there are several issues that have to be addressed for commercial development of products based on these materials. Our group focuses on the design and synthesis of novel conjugated polymers based on the heterocyclic building blocks benzobisazoles and benzodifurans. Polybenzobisazoles posses many exceptional electronic, optical and thermal properties, however the harsh conditions required for their synthesis and their lack of solubility in organic solvents limits their utility. Our group has developed a mild approach for the synthesis of benzobisoxazoles resulting in several building blocks suitable for designing new polymers. These materials have been used in both organic light emitting diodes and photovoltaic cells. In recent years, the electron-donating benzo[1,2-b:4,5-bprime;]dithiophene (BDT) moiety has been widely investigated for the synthesis of conjugated polymers due to its planar conjugated structure that facilitates πminus;π stacking, leading to good charge carrier mobility. As a result, PCEs approaching 8% have been obtained for BDT copolymers. Recently, our group has investigated the use of the benzo[1,2-b:4,5-b&’]difuran (BDF) as a building block for the synthesis of new conjugated polymers. These polymers exhibit enhanced solubility over the analogous benzodithiophene based materials, improving film formation and performance in organic photovoltaic cells. Additionally, our synthetic approach enables these monomers to be prepared in a few short steps and in high yield. Novel donor-acceptor copolymers can be synthesized by combining the BDF monomer with various electron deficient co-momoners. Our recent work on these materials will be presented.

The control of solid-state order is a key parameter to determine the electronic and photonic properties of organic compounds in a wide array of applications. Subtle changes in crystal packing can lead to dramatic improvements - or catastrophic decline - in device performance. Whilst control of intermolecular order is critical, myriad other parameters also depend on more general molecular functionalization, and in many cases there are no existing structure-function relationships to guide the materials designer. Do particular functional groups enhance or suppress singlet fission? What crystalline motifs or functional appendages can enhance thermal transport in an organic solid? How does functional group hydrophobicity impact film-forming properties? In this talk, I will discuss our latest investigations of the impact of functional group changes on a wide variety of electronic, optical and thermal properties of organic semiconducting chromophores.

Ammonia sensing attracts substantial research interest due to its potential association with chronic diseases like asthma, severe respiratory inflammation, and pneumonia arising from its presence as an environmental pollutant.^1,2 A simple device architecture is highly desirable for the purpose of fabrication, integration, and portability of sensors. Chemiresistors can realize such targets because they can be constructed only using three components including a thin and flexible substrate, a printable electrode, and a sensing material.³ Organic semiconductors (OSCs) have advantages like light weight, low cost, and ease of processing over inorganic counterparts and some of them have been used as active materials for fabricating ammonia sensors. However, OSCs reported so far for this purpose are generally p-type materials where exposure lowers sensor current owing to the electron-rich nature of ammonia.
In this contribution, we report an n-type OSC⁴ being used to construct chemiresistive ammonia sensors that present high current-increase sensitivity up to 0.5 ppm v/v for ammonia and good reproducibility. More importantly, when blending the n-type OSC with an easily obtained insulating polymer (polystyrene) the resulting sensors maintain comparable sensing properties with those containing pure active material, which favors lowered fabrication cost and improved printability of the devices, and opens routes to circuits requiring paired current-increase and current-decrease responses.
References:
1. E. Stokstad, Science 17, 238, (2014).
2. K. Besar, S. Yang, X. Guo, W. Huang, A. M. Rule, P. N. Breysse, I. J. Kymssis, H. E. Katz, Org. Electron.Under revision.
3. S.-W. Chiu, K.-T. Tang, Sensors13, 14214, (2013).
4. H. Zhong, J. Smith, S. Rossbauer, A. J. P. White, T. D. Anthopoulos, M. Heeney, Adv. Mater.24, 3205, (2012).

Novel electronics with wearable and disposable properties or other green elements can be achieved by utilizing various unconventional substrates such as paper, cloth, etc. However, limited by severe surface roughness and processing temperature of these substrates, it remains challenging to fabricate large numbers of elements to provide the desired functions with high yield rate and uniformity. In this work, we demonstrate a novel OTFT array fabrication method combining screen-printing, vapor phase deposition and laser drilling which can be directly applied on cellulose fiber substrates with roughness up to micrometer scale. Highly uniform 8×8 OTFT active-matrix array is developed and the average mobility and on/off ratio are 0.45 cm²V^-1s^-1 and 1.5×10⁸, respectively. Channel length smaller than 60 mu;m is highly reproducible by direct screen-printing the source/drain electrodes onto semiconductor surface. Thanks to the small overlapping of electrodes, the induced parasitic capacitance is small, and combining with the moderate carrier mobility, the devices can offer maximum operating frequency up to around 39 kHz, which is enough for large-area operations. By integrating 8×8 LEDs onto the transistors, the active matrix array can perform point scanning, line scanning and image display. With this array processing technique, active-matrix backplane, 2D optical sensor and pressure sensor or even memory array can be further developed to satisfy different flexible and green electronic application requirements.

Conjugated polymers have attracted attentions owing to potential application to flexible, cheap, and printable electronic devices such as organic photovoltaics (OPVs), light-emitting diodes (OLEDs), and field-effect transistors (OFETs) in the future. The donor-acceptor (D-A) motif in the backbone of the conjugated polymers is the state-of-the-art architecture to tune the energy levels of HOMO/LUMO and HOMO-LUMO energy gaps of the polymers, which are among the most important molecular parmeters in the design of organic semiconducting materials.
Recently, we have reported the synthesis and characterization of naphthodithiophenediimide (NDTI) as a versatile electron-acceptor unit for the p-functional materials (J. Am. Chem. Soc., 2013, 135, 11445-11448). The characteristic features of NDTI are: i) strong electron-accepting nature (E_LUMO ~ 4.0 eV); ii) a planar and rigid structure which are beneficial for carrier transport; iii) chemical flexibility owing to originating from the possible modification on the imide nitrogen atoms and at the thiophene a-positions. In this presentation, we describe the synthesis and characterization of various NDTI-based polymers, with different donor units or acceptor units, including benzo[c][1,2,5]thiadiazole (BTz) and naphtho[1,2-c:5,6-c&’]bis[1,2,5]thiadiazole (NTz).

Organic semiconductor thin films based on metal phthalocyanine and perylene structures are well known as promising materials for applications such as chemical sensors and solar cells. Electrical and optical parameters of those films are significantly affected by procedures and conditions of a thin films deposition process. Due to different conditions during the thin films deposition the planar organic molecules create different crystallographic alpha and beta phases with substantially different properties. These phase transitions can be established using X-ray and electron diffraction.
In this work we present complex properties study of thin films of zinc phthalocyanine, fluorinated zinc phthalocyanines (4F, 16F), perylene and PTCDA (Perylene-3,4,9,10-tetracarboxylic dianhydride) on fused silica and Si(100) prepared by organic molecular evaporation in high-vacuum deposition system (working pressure of 10^-5 Pa) and by pulsed laser deposition for comparison. We studied films prepared with various substrate temperature in the range from 20°C up to 400°C in terms of crystalline phase composition by XRD and SAED (selected area electron diffraction) and chemical composition by FTIR. We compare compositions of source materials with deposited thin films as well. On prepared films we investigated surface morphology by AFM and SEM and optical absorption in UV-VIS range. Finally we connect observed material properties with chemical gas sensor and optoelectronic applications.

Graphene is one of the potential alternatives to indium-tin-oxide (ITO) because of its very high mobility, ease of large-area fabrication and rare-earth free growing. However, in order to use graphene as an efficient electrode, its work function should be tuned to match the energy level of active layer. Hexaazatriphenyleneminus;hexacarbonitrile (HAT-CN) has been widely used to increase the work function of substrate and it enhances the charge injection.¹ In addition, recent studies have shown that HAT-CN can control the orientation of the molecules deposited on it. This molecular ordering can enhance the intermolecular charge transfers thus the power conversion efficiency (PCE) of organic photovoltaic devices could be improved.² Although the charge transfers should be strongly related to the electronic structures of the molecular film, fundamental studies on them are still lacking. Understanding the correlation between molecular ordering and electronic structures would be the key for high performance device.
In this study, in situ ultraviolet photoelectron spectroscopy (UPS) and x-ray photoelectron spectroscopy (XPS) measurements were carried out for the interfaces of CuPc/HAT-CN/graphene and CuPc/graphene. These measurements show the changes in electronic structures of CuPc overlayer with the insertion of HAT-CN. Molecular orientation of CuPc was also studied with backscattering-geometry Raman spectroscopy. The orientation of CuPc could be estimated through the Raman tensors analysis considering the point group symmetry of CuPc with respected to the molecular orientation on substrate. HAT-CN interlayer changes the orientation of CuPc significantly and it affects the interfacial electronic structures as well. The detailed correlation between the molecular orientation and electronic structures will be discussed.
¹ Christos Christodoulou, Angelos Giannakopoulos, Marco Vittorio Nardi, Giovanni Ligorio, Martin Oehzelt, Liping Chen, Luca Pasquali, Melanie Timpel, Angelo Giglia, and Stefano Nannarone, The Journal of Physical Chemistry C 118, 4784 (2014).
² K. S. Yook, B. D. Chin, J. Y. Lee, B. E. Lassiter, and S. R. Forrest, Appl Phys Lett 99 (2011).

From the starting point of organic electronics in the 1990's, steadily increasing effort is put into the improvement of organic solar cell (OSC) performance. This led to a tremendous growth rate of their photo conversion efficiency. The progress is based on new materials tailored for OSC applications as well as on advanced skills in device engineering. But many of the models and characterization techniques applied in the field of organic electronics are restricted to small clusters of molecules or model systems as individual interfaces only, thus lacking prediction when it comes to full devices. With scanning Kelvin probe microscopy (SKPM) accompanied by device simulations we try to access the physics of entire OSC devices and bridge the gap between the molecular and the macroscopic understanding. On this road, the nature of SKPM as a surface characterization method and of OSCs as horizontally layered devices poses an experimental challenge. In our measurements we gain access to the OSC cross sections by milling trenches into the devices with a focused ion beam (FIB).
Our scanning probe station is placed within the vacuum of a scanning electron microscopy (SEM)/FIB cross beam system. Under observation with the SEM we place the cantilever right at the FIB prepared cross section and investigate the potential distribution of the solar cell in situ with SKPM.
The SKPM measurements are backed by IV characterization and device simulations. Varying the parameter of the active layer/contact interface, we simulate their impact on the potential distribution and the device characteristics of the OSC. Experimentally, we translate the varying interface parameter by systematic contact modulation on F₄ZnPc/C₆₀ OSCs. In IV measurements and cross sectional SKPM measurements we check the validity of the applied models and identify loss mechanisms and their localization in the solar cell device.

Poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) is a well-known organic semi-conductor frequently used as anode and/or hole-injection layer in solution-processed organic light-emitting diodes (OLEDs) and organic solar cells (OSCs). In such devices the PEDOT:PSS layer is typically covered by one or more photoactive layers. If the latter contains either microscopic or macroscopic defects, device performance is severely compromised due undesirable low resistive contact between cathode and anode.
The current paper presents a highly effective repair procedure that prevents the occurrence of such short circuits and leakage currents, by local over-oxidation of the PEDOT. The approach involves a post-treatment of the active device stack with a water-borne oxidizer, which reacts with the PEDOT in areas where it has become (virtually) exposed. A theoretical model comprising a combination of General Effective Medium (GEM) theory and rate law kinetics will be presented, describing how the PEDOT:PSS conductivity depends on treatment time and oxidizer concentration. Fitting experimental data against this model not only provides a quantitative basis for repair treatment conditions, but also yields interesting information on the nano-morphology of PEDOT:PSS. Next, the repair procedure itself will be highlighted, showing how both as-processed and deliberately defected devices obtain full recovery of operational characteristics and efficiency.

Electrochromic materials that exhibit the necessary colors of RGB and CMY have been sought after by the display industry for the fabrication of low-power, thinner, and less complex displays. ECDs that exhibit neutral color transitions are of special interest in the current architectural and transportation glass as well as for eyewear. Neutral colors provide an unmitigated view of the environment without distorting original colors. Their low brightness minimizes eye fatigue and generates a calm, non-distractive atmosphere.
Unlike the traditional approach for achieving a specific color, we present a facile, one-step method to color tune electrochromic devices (ECDs) that switch between two neutral colors via an in situ electrochemical polymerization of electroactive monomers such as 3,4-ethylenedioxythiophene (EDOT) in the presence of a small molecule yellow solvent dye and hence neutral coloration is achieved using all commercially available materials sold in relatively large abundance. The in situ method offers a flexible, one-step procedure resembling that of a lamination process that for rapid screening of different electrochromic color transitions. Devices prepared from EDOT and a solvent yellow dye exhibited photopic contrasts of ca. 30% without background correction when assembled with flexible indium doped tin oxide polyethylene terephthalate substrates. In addition, devices exhibited switching speeds as low as 2 seconds, and great color uniformity. Large, defect-free ECDs of 100 cm² were fabricated, characterized and tested. This size exceeds the needs for the active switch area for goggles, lenses, mirror and small display applications.

Conformal, flexible, wearable, and stretchable electronics will have a significant impact in the years to come as displays will be decorating garments worn as well as the sides of skyscrapers and the furniture we sit on. Electrochromics, a color change controlled by an electrical stimulus, is one technology being explored for numerous applications and has already been successfully commercialized in the arena of transportation glass. As opposed to rigid glass substrates, more recent studies have been devoted to plastic, flexible substrates for their potential application in products such as color-transitioning eyewear, smart windows, and textile. To our knowledge, no commercially available flexible electrochromic devices are currently mass produced due to numerous challenges, such as cost of manufacture, device stability/ruggedness, and performance.
Utilizing our in situ method in which an all-in-one solution is applied between two substrates, and a monomer in that solution is later electrochemically coupled to make conjugated polymer after the solution has been converted to a polyelectrolyte gel via UV or thermal crosslinking, we undertook a study to optimize device perfomance via the optimization of the component ratio in the all-in-one solution. Gel polymer electrolytes based on poly(ethylene glycol) dimethacrylate (PEGDMA) and poly(ethylene glycol) methyl ether acrylate (PEGMA), containing lithium trifluoromethanesulfonate (LiTRIF) as the salt and propylene carbonate (PC) as plasticizer, were used for the study; we investigated varying electrolyte parameters, including salt weight percent loading, mono/diacrylate functional PEG weight ratio, and the plasticizer to PEG weight ratio. We set forth to optimize the resulting gels to exceed the flexibility of commercial indium doped tin oxide coated polyethylene terephthalate substrates in maximum bending radius of curvature, exhibit an ionic conductivity up to 1.36*10^-3 S/cm, and yield electrochromic devices (ECDs) with photopic contrasts as high as 53% (without background correction) using poly(2,2 dimethyl-3,4-propylenedioxythiophene) (PProDOT-Me₂) as the standard electrochromic material. In addition to ionic conductivity, the crosslink density of the gel polymer electrolyte was found to play an important role on photopic contrast of resulting ECDs. Using these results, a 150 cm² defect-free neutral grey flexible EC goggle insert and a 110 cm² flexible ECD with high resolution patterns were assembled and tested.

One of the main factors limiting the power efficiency of organic electronic devices such as organic light emitting diodes (OLEDs) is the low intrinsic conductivity of the hole transport as well as electron transport layers. One possibility to increase the conductivity of these layers is to employ electrochemically doped layers. It has already been shown that the conductivity of these transport layers can be increased by several orders of magnitude by doping [1]. However, especially in the case of p-type doping using transition metal oxides as acceptor molecules, the doping efficiency has been reported to be in the range of only a few percent, leading to high doping concentrations [2]. The reasons for this low doping efficiency as well as the charge transfer process in general are still not fully understood.
Infrared (IR) spectroscopy is very sensitive to charge transfer in doped layers, as the charged molecules that form in the doped layers exhibit a different vibrational spectrum compared to the neutral molecules [3].
We investigated p-type doping of the ambipolar charge transport material 4,4&’-Bis(N-carbazolyl)-1,1&’-biphenyl (CBP) using the transition metal oxides MoO₃ and WO₃ as acceptor materials, as well as n-type doping of CBP using cesium carbonate (Cs₂CO₃).
In the spectra of the doped layers, strong changes can be observed that show the formation of charged CBP molecules. Employing a fitting procedure, we were able to derive the IR spectra of the charged CBP molecules. The spectra of the neutral CBP, the CBP cation (derived from p-type doping) as well as the CBP anion (derived from n-type doping) are in agreement to the spectrum of CBP⁰, CBP⁺¹, respectively CBP^-1 that were calculated based on density functional theory.
In the spectra of the p-doped layers, the appearance of a broad electronic excitation shows the formation of hybrid orbitals. While the vibrational spectrum of the CBP cation is independent of the acceptor molecule that is used, the energetic position of the broad excitation differs for the two different acceptor molecules.
In the spectra of the n-type doped layers this broad excitation does not appear. Using a mass spectrometer, we could show that Cs₂CO₃ decomposes during the evaporation process. We could also show that the doping effect disappears when the n-type doped sample is exposed to air, most probably due to an oxidation of the decomposed dopant.
Financial support by BMBF via MESOMERIE Project (FKZ 13N10724) is gratefully acknowledged.
References
[1] M. Kröger, S. Hamwi, J. Meyer, T. Riedl, W. Kowalsky, and A. Kahn, Org. Electron. 10, 932-938 (2009).
[2] S. Hamwi, J. Meyer, T. Winkler, T. Riedl, and W. Kowalsky, Appl. Phys Lett. 94, 253307 (2009).
[3] T. Glaser, S. Beck, B. Lunkenheimer, D. Donhauser, A. Köhn, M. Kröger and A. Pucci, Org. Electron. 14, 575-583 (2013).

This study is the first report of stereochemically pure cis and trans 1,3-substituted poly(propylenedioxythiophene)s PProDOTs. Herein, pure stereoisomers of 1,3-diisopropyl-3,4-propylenedioxthiophene are compared with varying ratios of the two stereoisomers to evaluate structure-property relationships as it pertains to visible absorption, color, and electrochemical redox potentials. In addition, high-level calculations were performed to support the experimental data, and to gain an understanding of the structure-property relationship of the polymers. A 90 nm blue shift in the visible lambda max is indicative that the trans 1,3-substituted PProDOT has disruption of conjugation based upon interactions with one of the substituents of the 7-membered ring.

The diffusion of two monomers inside of a solid-state gel electrolyte and their electrochemical copolymerization is applied as a method to match the monomer feed ratio to a specific color that results from a conjugated copolymer having a single absorption in the visible region. In this study, several different monomers were evaluated in which the conjugated homopolymers have a single wavelength absorption with a focus on 1,3-di-tert-butyl-3,4-propylenedioxythiophene (ProDOT-tBu₂) and 2,2-dimethyl-3,4-propylenedioxythiophene (ProDOT-Me₂), since their respective homopolymers are at the high and low energy extremes of the visible spectrum. The combination of the two monomers generates a continuum of subtractive colors of any single wavelength in a solid-state electrochromic device in the colored state and when the device is switched to the bleached state, this spectrum of colors all transition to transmissive sky blue when oxidized. Similar to the paint industry, where two or three dyes can be combined to make any color, various feed ratios of only two or three different monomers in the synthesis of conjugated copolymers (CPs) could yield the same result for electrochromic applications. This study could have a significant impact on electrochromic applications such as eyewear, displays, windows, and fabric wherein achieving a specific color or color set is critical to its functional use.

Organic-based thin film electronics have received much attention owing to their flexibility, transparency, and low-cost. Due to discontinuity in morphology, interfacial dipole barrier, and Schottky barrier at electrode/organic interface, convenetional metal electrodes are not favorable for high performance organic thin-film transistors (OFFTs). Graphene has been suggested as a hole-injecting electrode material due to its high work function, extraordinary electronic properties and strong π-π interaction with organic molecule; all of which can reduce the injection barrier at the electrode/organic interface. In particular, due to its solubility, large quantities produce, and its chemical functionality, reduced graphene oxide (RGO) has been introduced as an promising electrode for OFETs. Its tunability of electrical and optical properties can make RGO a highly desired electrode material because the work function match is essential for better charge injection at electrode/organic interface.
To understand the impact of reduction effieicncy of RGO, it is crucial to investigate the electrical properties of the OFETs as a function of different reduction effieincy of RGO electrodes. In this talk, we will discuss the performance of OFETs using different reduction efficiency of RGO electrodes. We show that the performance of pentacene transistors can be improved compared to conventional metal electrodes and tuned by varying the carbon sp² fraction (or reduction efficiency) of RGO electrodes.

While progress has been made in the area of polymer based solar cells, there is still much ambiguity. One of the main topics of interest is the degree of crystallinity of the semiconducting polymer. Differential scanning calorimetry (DSC) makes it possible to measure absolute polymer crystallinity with knowledge of the enthalpy of fusion of a perfect crystal (ΔH_m^infin;) of the polymer being investigated. In this study, ΔH_m^infin; of the semiconducting polymer, poly(3-hexylthiophene) (P3HT) was determined using DSC with a correction factor based on the crystalline content determined by grazing incidence small angle scattering (GISAXS). With this technique, the value of ΔH_m^infin; = 42.9 ± 2 J/g was attained. This value was then utilized to investigate P3HT crystallinity in bulk heterojunction films containing 50 wt.% phenyl-C₆₁-butyric acid methyl ester (PCBM) with interesting results. Temperature modulated DSC (MDSC) was used to reveal weak and overlapping transitions in the polymer blend. It was observed that, upon examination of both the crystallinity and glass transition temperature (T_g), the morphology of a traditional drop cast sample is different to that of a spin cast sample, which is the typical device processing technique. Furthermore, the spin cast sample as prepared, showed an essentially amorphous P3HT morphology, while thermal annealing increased it to approximately 30% crystallinity with a complimentary 4°C T_g increase. The T_g increase can also be used to describe the composition of the amorphous regions in the film, as will be discussed in the oral presentation. The results of this study offer direct evidence of the relationship between the morphology of P3HT and the increase in device performance observed after annealing. Therefore, DSC and MDSC offer a unique insight into the morphology of polymer based solar cells in terms of the polymer crystallinity and composition of the amorphous phase.

Optical and structural properties of ultrathin films and isolated molecules of poly (9,9 dioctylfluorene) (PFO) are investigated in this work. Ultrathin films (<10 nm) and isolated molecules were deposited by spin-coating on top of quartz substrates having extremely flat surfaces under conditions of very low molecular dilution in chloroform. We employed three spectroscopic techniques to characterize such ultrathin films, namely: confocal fluorescence microscopy equipped with spectral and time-lapse fluorescence imaging facilities, UV/Vis absorption spectroscopy and temperature dependent photoluminescence spectroscopy. We observed a gradual planarization (β phase) of the fluorine repeat units with the decrease of PFO film thickness in the range between 5 nm and 0,7 nm at room temperature, even though the β phase is not expected when chloroform is used as solvent. Low temperature (5 K) luminescence of these ultra thin films reveals a well resolved zero-phonon emission band between the energies of the planar phase (2,80 eV) and the amorphous phase (2,91 eV) that was associated to perturbations of the planar conformation by interchain interactions. However, the emission of these intermediated states are not observed for film thicknesses higher than 5 nm, which suggest that they behave as light-harvesting states responsible for guiding the excited state from higher energy non planar to lower energy planar phase states via energy transfer process. In addition, isolated PFO molecules probed by the confocal microscope allowed the insight into mechanisms associated to the planarization of the polymeric chain induced by the substrate surface. Single PFO molecules have frozen β phase structure at room temperature on the quartz substrate. This structure is disturbed by the atomic vibrations of the substrate that broaden the emission lines without breaking the rigid planar conformation imposed by the substrate. Modifications molecular conformation by changing the temperature above the glassy transition or the strength of substrate/PFO interactions will be presented.

The planar conformation assumed by polyfluorenes is used as a model system to elucidate how solid surfaces interact with an individual polymer chain and to access the effects of substrates and interchain interactions on the polymer conformation. We demonstrate that confocal fluorescence microscopy is very sensitive for characterizing photophysical processes associated to conformation assumed by a single conjugated unit of poly(9,9 dioctylfluorene), or PFO, in contact with substrate surfaces. Polymer samples were produced on top of quartz substrates by spin-coating from PFO highly diluted chloroform solutions. We correlate the energy of the zero phonon emission of the PFO planar e non-planar (helical) conformations to the extent of the surface-polymer interaction. In the case of single molecule, the planar phase (lower energy) is induced by the strong surface-polymer interaction in the case of charged hydrophilic surface. Moreover, this interaction can be strongly perturbed or entirely disrupted by interchain interactions involving two or more polymer chains. As a consequence, emissions in isolated molecules have been resolved for the first time between the energies of the planar and the non-planar phases that were associated to perturbations of the planar structure. We found that the planar conformation sets a lower limit to the PFO zero phonon energy. In addition, PFO isolated molecules have frozen planar conjugated units at room temperature on hydrophilic quartz substrate. Thermal vibrations are not enough to disrupt p conjugation in such one dimensional structure due the strong surface interactions and probably to steric effects, but they have strongly impact on the exciton line width.

We demonstrate a high open circuit voltage (V_oc), high fill factor (FF), amorphous single-junction organic photovoltaic cell consisting of tetraphenyldibenzoperiflanthene (DBP) as donor and pyrrolo[3,4-c]pyrrole- 1,4-dione, 3,6-bis(4-chlorophenyl)-2, 5-dihydro (DPP) as acceptor. This is the first time that DPP is used as the acceptor in a vapor-deposited, fullerene free organic solar cell.
Under one sun, the solar cell exhibits a power conversion efficiency (PCE) of 4.7±0.2% with a V_oc of 1.22 V, a short circuit current density (J_sc) of 6.4 mA/cm², and a FF above 0.6. To the best of our knowledge, our achieved V_oc is the highest value ever reported for a single-junction organic solar cell. The PCE of our device is higher than that of the best DBP/ C₆₀ cells ever reported, suggesting that DPP can work as an efficient alternative for fullerene.
Our DBP/DPP diodes demonstrate low dark injection current, high rectification values and linearly proportional with respect to incident light intensity. Based on small perturbation transient photovoltage measurements, we hypothesize that space charge does not accumulate in the solar cell, which results in the high and FF values due to minimized bimolecular recombination events. Using X-ray diffraction, we observed that both DBP and DPP films are amorphous, which shows that it is actually possible to achieve a high PCE of solar cells by using amorphous vapor-deposited materials. Preliminary efforts towards incorporating this high efficiency single-junction cell into multijunction device architectures are also described.

Organic thin film transistors (OTFTs) based on polymer semiconductors attracted much attention due to the advantages of flexibility, light weight and low cost. Their low carrier mobility has been an issue for many applications. It is generally accepted that the intermolecular hopping of charge carriers between polymer chains is the rate-determining step for charge transport, and thus a highly ordered crystalline structure is preferred for efficient charge transport. Accordingly, highly symmetric and regular molecular structures have been adopted to improve the molecular packing order and the charge transport performance. In this study, we reported an interesting observation that the presence of an appropriate amount of oligomers in a diketopyrrolopyrrole(DPP)-based conjugated polymer could significantly increase the crystallinity of the polymer film and improve the grain connectivity of the thin films, resulting in dramatically enhanced charge transport property. It is considered that the presence of these oligomers could promote the motion of high molecular weight polymer chains to form highly crystalline grains and also bridge the grain boundaries to establish well interconnected polymer networks for highly efficient charge transport.
sect; This work is published in J. Mater. Chem. C. 2013, 1, 4423.

A fundamental understanding of how to access different polymorphs within organic semiconductor thin films is critical for device optimization. Polymorphism results when molecules pack together in different motifs, of which one may be more favorable for charge transport compared to others. In solution-grown single crystals, core-chlorinated naphthalene tetracarboxylic diimide (NTCDI-1) readily adopts the P2₁/c crystal structure (polymorph 1); field-effect transistors comprising single crystals of this polymorph exhibit high electron mobility (8.6 cm²V^-1s^-1) [1,2]. We report a previously unobserved polymorph (polymorph 2) of NTCDI-1 when single crystals are grown by physical vapor transport. While polymorph 1 forms large needle-like single crystals, polymorph 2 exhibits two-dimensional growth, resulting in sheet-like crystals. Single-crystal field-effect transistors were fabricated with each polymorph as the active layer; preliminary results indicate devices comprised of polymorph 2 exhibit higher electron mobilities compared to devices containing polymorph 1. This result is consistent with the general suggestion that two-dimensional crystals are more conducive to lateral charge transport than one-dimensional crystals. Upon thermal evaporation, NTCDI-1 forms polycrystalline thin films of exclusively polymorph 2 on silicon oxide and octadecyltrichlorosilane modified-silicon oxide substrates. These films transition from polymorph 2 to polymorph 1 when exposed to vapors of chlorinated solvents, such as chloroform, dichloromethane, and dichloroethane. Conversely, thermal evaporation of NTCDI-1 on hexamethyldisilazane modified-silicon oxide substrates yields polycrystalline thin films of mixed polymorphs. When these films are exposed to tetrahydrofuran, portions of the films that adopt polymorph 1 can be readily converted to polymorph 2. This tunability and accessibility of the different polymorphs post-deposition will allow us to examine the kinetics and mechanism of transformation closely.
[1] Oh, JH, Suraru, SL, Lee, WY, Konemann, M, Hoffken, HW, Roger, C, Schmidt, R, Chung, Y, Chen, WC, Wurthner, F, and Bao, Z. “High-Performance Air-Stable n-Type Organic Transistors Based on Core-Chlorinated Naphthalene Tetracarboxylic Diimides.” Adv. Funct. Mater.2010, 20, 2148-2136.
[2] He, T, Stolte, M. and Wurthner, F. “Air-Stable n-Channel Organic Single Crystal Field-Effect Transistors Based on Microribbons of Core-Chlorinated Naphthalene Diimide.” Adv. Mater.2013, 25, 6951-6955.

Solution-processability is a key advantage of organic semiconductors, enabling us to easily deposit these materials at low-cost. Among organic semiconductors, small molecule semiconductors are highly crystalline and thus exhibit high carrier mobilities; however, they present challenges in forming uniform films by solution-processing. Previously, we have shown that the proper choice of polymer binder allows for not only producing uniform thin films of 5,6,11,12-tetraphenylnaphthacene (rubrene) / polymer blends by spin-coating, but also controlling the microstructure and electronic properties of such films. In this work, we investigate casting solvent effects on phase separation, crystallization, and resulting electronic properties of rubrene / polymer blend films. Blend of rubrene and insulating polymer, polystyrene (PS) were dissolved in three different solvents: o-dichlorobenzene (o-DCB), toluene, and chloroform. The blend solution was spin-cast and then annealed at 170 0C to induce crystallization of rubrene. The morphologies and crystalline phase of rubrene in blend films are found to be strongly influenced by the choice of solvent. For instance, rubrene / PS blend films of o-DCB and toluene mostly exhibit spherulitic morphologies with orthorhombic phase, while rubrene / PS thin film of chloroform have dendritic and spherulitic morphologies consisting of triclinic and orthorhombic phase, respectively. To measure the electronic properties of the rubrene / polymer, bottom-gate and top-contact field-effect transistors were fabricated. Among the three blend films, rubrene / PS blend films of o-DCB exhibit the highest field-effect mobilities (mu;_ave = 0.5 cm² V^-1 s^-1), while in the case of transistors based on blend film of chloroform, only orthorhombic phase exhibit transistors behavior with lower carrier mobilities.

Organic field-effect transistors (OFETs) that operate with good electrical stability were prepared by synthesizing fluorinated polyimide (PI) gate dielectrics based on 6FDA-PDA-PDA PI and 6FDA-CF3Bz-PDA PI. 6FDA-PDA-PDA PI and 6FDA-CF3Bz-PDA PI contain 6 and 18 fluorine atoms per repeat unit, respectively. These fluorinated polymers provided smooth surface topographies and surface energies that decreased as the number of fluorine atoms in the polymer backbone increased. These properties led to a better crystalline morphology in the semiconductor film grown over their surfaces. The number of fluorine atoms in the PI backbone increased, the field-effect mobility improved, and the threshold voltage shifted toward positive values (from minus;0.38 V to +2.21 V) in the OFETs with vacuum-deposited pentacene and solution-processed triethylsilylethynyl anthradithiophene. In addition, the highly fluorinated polyimide dielectric showed negligible hysteresis and a notable gate bias stability under both a N₂ environment and ambient air (25% and 55% relative humidity). This study revealed that OFETs containing fluorine groups in the gate dielectrics operated stably under a variety of conditions because they prevented the creation of semiconductor/dielectric interface traps.

Electrochemical polymerization is a method for preparing p-conjugated polymers as films on surfaces of electrode. Our group previously reported an approach to control microscopic structure by electrochemical polymerization using liquid crystalline media. This method allows us to transcribe microscopic structure of the liquid crystalline media to obtained films.
Herein, we synthesized novel menthol derivatives and measured their properties. By adding this product to nematic liquid crystal, they induce cholesteric liquid crystal. They possess large helical twisting power in nematic liquid crystal, so we can call this molecule chiral inducer in NLC. In addition, they possess homeotropic orientation in smecticA liquid crystal (SmA), so we also can call this molecule homeotropic inducer in smecticA. Electrochemical polymerizations in cholesteric liquid crystal (CLC) and smectic liquid crystal (SmLC) were conducted.
Helical twisting power of inducers was estimated by Grandjean-Cano wedge cell method. The b_M of the (+)- or (-)- inducer is 41.6 and 38.8 mm^-1, respectively. This large helical twisting power is probably due to the number of asymmetric carbons and the bulky structure of menthyl group.
The polymer films prepared in CLC using (+)- or (-)- inducers showed mirror image Cotton effect in reduced state. This indicates that the polymers forms helical aggregation. The POM image of the polymer films prepared in CLC shows the narrow-pitched fingerprint texture, which is very similar to that of CLC.
On the other hand, the POM image of the polymer films prepared in SmLC shows the texture that is very similar to that of SmLC.
These results indicate the structure of liquid crystal media can induce the structure of polymer films, which is very similar to liquid crystal media.

Electrochemical polymerization is a method for preparing p-conjugated polymers as films on surfaces of electrode. Our group previously reported an approach to control microscopic structure by electrochemical polymerization using liquid crystalline media. This method allows us to transcribe microscopic structure of the liquid crystalline media to obtained films. In this study, we adopted smectic liquid crystal that has layer structure and relatively high anisotropic orientation. Moreover, magnetic field was applied parallel or perpendicular to the substrate during polymerization to obtain uniaxial polymer films.
Herein, we measured the surface morphology of the polymer films by polarizing optical microscopy (POM) and scanning electron microscopy (SEM). The polymer film prepared under the parallel magnetic field forms mono-domain, where fibrils uniaxially grows vertically to the magnetic field. This fibril growth occurred at layer boundary of smectic liquid crystal according to the different growing speed between layer boundary and intra-layer. On the other hand, the polymer film prepared under perpendicular magnetic field forms multi-domain, where many circular fibril growth was observed. This fibril growth also occurred at the layer boundary of smectic liquid crystal.

One of factors determining the properties of organic electronic devices such as organic field-effect transistors (OFETs) is a charge-injection barrier from electrodes into organic semiconductors. Self-assembled monolayers (SAMs) are often used as a modification layer of electrode surfaces to tune the charge-injection barrier. However, if the SAMs have a disordered structure, the charge-injection barrier can be modulated by electric-field-induced structural switching of the SAMs [1]. By exploiting this switchable nature, we have achieved a reversible polarity reversion of metal/organic-semiconductor/metal diodes with a switching ratio as high as 10⁵ [2]. In this presentation, the effect of permanent dipoles of the SAMs will be discussed. The magnitude and orientation of the dipoles is found to largely affect the switching ratio.
[1] R. Nouchi and Y. Kubozono, Org. Electron. 11, 1025 (2010).
[2] R. Nouchi, M Shigeno, N. Yamada, T Nishino, K Tanigaki and M. Yamaguchi, Appl. Phys. 104, 013308 (2014).

Because of a better structural ordering for the organic single crystal, the single-crystal OFET has the higher carrier mobility than the poly-crystal OFET. Therefore, the single-crystal OFET can be applied not only in the fundamental research, but also in the high-speed organic flexible electronics. However, the key issue for the high performance single-crystal OFET consists of the achievement of large-sized organic crystal and the challenges of device processing such as the interface contamination and carrier injection from electrodes. This work focuses on the achievement of high mobility OFET fabricated using the organic single crystal and an interface controlling. We demonstrated a new strategy to synthesize large pentacene single crystal, which consists of initial crystallization from pentacene monolayer grains to nanosized single crystals and subsequent physical vapor transport growth on the nanocrystals as seeds. We proposed the crystal growth diagram based on the thermodynamics in this two-step growth process [1] and also clarified the optimum technique conditions in fabricating the crystal devices.
Firstly, to optimize the crystallization conditions, we have systematically studied the relationship between the molecular nucleation and growth conditions and the parameters of deposition with the physical vapor transport method. Secondly, a novel growth method based on “two step growth method”, namely, initial crystallization from small molecular monolayer grains to nanosized single crystals and subsequent PVT growth on nanocrystals as seeds has been proposed to fabricate larger micro-sized crystals with a better interface quality. Thirdly, an optimal device fabrication process [2] is proposed to achieve the high mobility OFET using an organic single crystal. The carrier mobilities are larger than the corresponding poly-crystal film devices and showed apparent correlation with the thickness of crystal and orientation dependence. A temperature dependent measurement for single crystal device was also carried out to elucidate the carrier transport properties. The deep understanding of transport mechanism for single crystal OFET may be helpful for further optimizing the design of organic single-crystal transistors for a better device performance.
References:
[1] Qiao Jin, Dexing Li, Qiong Qi, Yiwei Zhang, Jun He, and Chao Jiang*, Cryst. Growth Des., 12(2012), 5432-5438.
[2] M. Mirza, J. W. Wang, D. Li, S. A. Arabi, and C. Jiang* ACS Appl. Mater. Interfaces, 2014, 6 (8), pp 5679-5684.

Thermal treatments can induce the migration or stabilization of the chemical components of the organic light emitting diodes (OLEDs), resulting in the sensitive change of the device performance. Considering the lifetime problems in the OLEDs. Organic light emitting diode (OLED) has many advantages like colour reproducibility, relative thin layer thickness and low power consumption. But it also has disadvantages. One of general problems of OLED devices is durability. So, Toto see the influence of examine the dependence of thermal stress, We gave the green OLED device thermal stress for looking the temperature-degradation characteristics. the change of Time dependence of thermal stress is analyzed. I-V, C-V characteristics and impedance spectroscopy are monitored measuredsimultaneously. The current level of stressed device decreased is lowed compared by pristine devices identified by the increase of the ideality factor. Thermally stressed device are compared with pristine device. The impedance spectroscopy analyzed by the cole-cole plot or the equivalent circuit model indicated the change of the quality of the interface layer of OLEDs, correlated with the thermal effects on the interfaces.Impedance difference between pristine and stressed devices is analyzed by cole-cole plot. The half circle shape of stressed one is larger than reference one. And C-V difference also measured. Compared with the pristine device which has sharp peak, thermally stressed one has a smoother curve.

In this paper, OLEDs with high power efficiency using various sizes of nano and micro random pattern were demonstrated. By different ZnO composition, wet etching patterns showed different morphology and size. Original etched patterns were transferred to glass substrate using direct nano-imprinting lithography with HSQ for using thin IZO electrode with low sheet resistance. Phosphorescent OLEDs on both patterns showed high power efficiency of 105 (nano) and 129 (micro) lm/w respectively, also increased external quantum efficiency with 28 and 58%, and 94 and 132% with hemi-sphere lens in comparison with reference device. We expect nano pattern, which had low haze, can be applied in flat panel display and micro pattern, which had high haze but large enhancement ratio in efficiency, can be applied in solid state lightings.

Polymer/silicon heterojunction solar cells based on crystalline n-type silicon readily exceeded efficiencies of 10 % with the polymers PEDOT:PSS^[1] and P3HT^[2]. In combination with the possibility to use solution processing to form an electron-hole separating junction, these hybrid devices become highly attractive. As a consequence a lot of research has been devoted to polymers as an alternative emitter material. Here, we present a study in which the influence of the organic layer on the device performance is investigated with three different thiophene based emitter layers. The molecular motifs of the applied materials, poly(3-hexylthiophene-2,5-diyl) (P3HT), poly(3-[3,6-dioxaheptyl]-thiophene) (P3DOT), and poly(3-[2,5,8-trioxanonyl]-thiophene) (P3TOT) differ in the amount of ether groups in the alkyl side chains of the organic semiconductors. The resulting structural differences were investigated using AFM, UV/VIS and XRD measurements. For the best devices the power conversion efficiencies exceeded 10 % with open circuit voltages reaching values higher than V_OC= 650 mV. Please note that no light trapping structures were used and that these values are comparable to those of diffused Si emitter cells. Furthermore, spectral response measurements were conducted and compared to conventional amorphous silicon/crystalline silicon (a-Si:H/c-Si) solar cells with a similar layer stack. Conclusions regarding reduced parasitic absorption, charge carrier generation and passivation of the interface in the hybrid solar cells can be drawn from the spectral response measurements by varying the passivation layers in the a Si:H/c-Si solar cells (SiO_x or intrinsic amorphous Si). As a last step, first results on electrochemically modified silicon interfaces were included in the device architecture, which is a promising approach for further optimization.

Organic Light-Emitting Diodes(OLEDs) have recently started to arouse much interest in various applications including mobile devices, solid state lightings and flat panel displays. OLEDs show many advantages such as high luminous efficiency, relative low power consumption and adaptability to flexible substrates. However, low light extraction efficiency needs to be improved for OLEDs to be used in such applications. Light extraction in the conventional OLEDs is limited to ~20% because some of the light generated in the active layer is confined in the device by the interface between the organic layers, electrode and glass substrate which have different refractive indices. There have been many attempts to resolve the low light extraction efficiency by introducing random texturing layer, photonic crystals, micro-lens arrays and modified substrates.
In this work, we have demonstrated a cost effective and easy fabrication process to resolve the low light extraction efficiency by investigating the dielectric-metal-dielectric(DMD) multilayer stacked electrode using solution processed tantalum pentoxide(Ta₂O₅), Au and molebdenum trioxide(MoO₃). MoO₃ was used to improve the charge injection and the Ta₂O₅ was used to tune the optical micro-cavity without affecting the electrical characteristics of the OLED device.
The sol-gel processed Ta₂O₅ was spin-coated at relatively high velocity and the thin film exhibited refractive index variation according to the thermal annealing temperature. High refractive index thin film was formed improving the light extraction at an optimum condition. The optimization process was carried out by simulation with transfer matrix method and a green phosphorescent device was fabricated using tris[2-phenylpyridinato-C2]iridium(#8546;) (Ir(ppy)₃) doped in 4prime;-bis(carbazol-9-yl)biphenyl (CBP). The OLED device with ITO exhibited external quantum efficiency(EQE) of 13.9% at 1000cd/m², whereas the device with multilayer stacked electrode exhibited EQE of 21.1% at 1000cd/m², indicating 52% enhancement in light extraction efficiency.

Solution-processable conjugated polymers are viable semiconductors for the fabrication of cheap electronic devices on flexible substrates. Despite their semicrystalline nature, thienothiophene (TT) and diketopyrrolopyrrole (DPP) based copolymers have recently shown exceptional charge carrier mobilities due to their rigid conjugated backbones that allow a relatively high amount of ordering in the solid state. While the crystallinity of a polymer is known to depend strongly on its molecular weight (MW), typical polymers made with TT of DPP monomers have polydispersity indexes (PDIs) around 2.0, indicating that chains with significantly different lengths are present. Herein, by precisely selecting polymer chain lengths (through preparative size exclusion chromatography) characterizing the resulting thin film microstructure (by AFM, Synchrotron GIXS), and measuring the electronic properties via TFTs, we describe new insights into the relationship between chain ordering and charge transport.¹ Specifically, the roles of side chain and backbone crystallization and chain entanglement are examined. We further show it is possible to drastically improve the mobility in poorly performing films prepared from low MW polymers by adding only small amounts of high MW material, suggesting that long chains strongly affect the intermolecular charge transport at grain boundaries by electronically bridging crystalline aggregates. The factors of molecular weight and PDI are accordingly found to affect morphology and performance of BHJ OPVs made with C60 derivatives. Indeed by optimizing MW and processing conditions we succeeded in increasing the PCE for PBTTT-C₁₄:PCBM devices by a factor of 2 over the previous state-of-the-art. Finally, in an effort to unravel the roles of inter- and intra-chain transport on the overall mobility we present a prototype polymer made of short, low PDI and fully conjugated segments linked by flexible aliphatic chains. Our flexibly-linked (FL) material exhibited improved thin-film formation compared to the low MW starting polymer and unique thermal properties which were exploited to demonstrate the evolution between distinct thin film morphologies without altering the chain length. Subsequent electronic characterization of the thin films revealed unexpected differences between the charge carrier transport and allowed a relative assessment of the roles of inter- and intra-molecular charge transport in the film.² Overall we show how our bottom-up approach to control crystallinity of both photovoltaic and transistor devices affords deep insights and directs the design of the next generation polymers.
1. A. Gasperini, K. Sivula Macromolecules2013, 46, 9349minus;9358
2. A.Gasperini, S. Bivaud, K. Sivula under revision 2014

High triplet energy n-type dopants, lithium 2-(oxazol-2-yl)phenolate (LiOx) and lithium 2-(1-methyl-1H-imidazol-2-yl)phenolate (LiIm), were synthesized as n-type doping materials to reduce the driving voltage and increase the quantum efficiency of phosphorescent organic light-emitting diodes, LiOx and LiIm showed triplet energy of 2.65 eV and 2.82 eV for application as n-type dopants for green and blue phosphorescent organic light emitting diodes. The effect of the n-type doping materials, on the electron mobility and device performances of the phosphorescent organic light-emitting diodes was investigated. Electron mobility of electron transport was increased from 1 × 10^-4cm²/Vmiddot;s to 6.6 × 10^-4cm²/Vmiddot;s by doping LiOx and LiIm as n-type dopants. In addition, quantum efficiency of green and blue PHOLEDS with LiOx and LiIm doped electron transport layer was enhanced by 50%, 22% than that of the devices with Liq doped electron transport layer. It was revealed that Lithium complex can be used as high triplet n-type dopants for green and blue PHOLEDs for the first time.

Conjugated polymer-based light-emitting devices (PLEDs) have attracted a great amount of attention due to their low manufacturing cost, large area application and flexibility [1]. However, there are still many problems remaining which limit PLED efficiencies from approaching the intrinsic maximum internal quantum efficiency of the active material: (i) the poor controllability of the active layer&’s charge-carrier mobility (especially in nanostructured environments), which depends on the polymer chain organization and conformation [2]; (ii) the low light-extraction efficiencies (< 20%) [4] associated with the device architecture [3]; (iii) the low formation efficiency of the singlet exciton state [5]; and (iv) the slow radiative decay rate of the triplet exciton state [6]. Here, we employ polymer/nanoporous metal film (NPM) systems as a platform to study and control conjugated polymer chain alignment in nanoconfined environments and to improve light-extraction efficiency and, potentially, triplet radiative decay rate.
In our work, we developed a dewetting method to fabricate nanoporous silver (NPAg) with different porosities and pore sizes over large areas by controlling annealing temperature and time. NPAg thin films (50 nm -100 nm thick) with average pore area ranging from 0.01 mu;m² to 0.27 mu;m² and porosity ranging from 17% to 66% over 2 cm² areas were prepared. The light-emitting conjugated polymer poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) was deposited on the NPAg surface using drop casting methods. From grazing-incidence, wide-angle X-ray scattering measurements of F8BT on NPAg films, we observed that smaller pores facilitate a greater degree of π-π stacking in the in-plane direction. Poly(3-hexylthiophene-2,5-diyl) (P3HT)/NPAg and polyfluorene (PFO)/NPAg systems have also been studied to determine whether the influence of nanopores on the chain alignment of different kinds of polymer is the same or is dependent on the properties of the polymer. In addition, the scattering spectra of bare NPAg films showed very clear peaks between 400 nm and 500 nm and transmission decreased as the porosities decreased. Angle-dependent photoluminescence measurements will be employed to study the differences in light extraction efficiency between polymer-coated NPAg, polymer-coated planar Ag and polymer-coated glass to investigate whether the light scattering properties of the NPAg structure can increase the light-extraction efficiency of the system.
[1] S. Kappaun, et al. Int. J. Mol. Sci. 2008, 9, 1527.
[2] M. Aryal, et al. ACS Nano. 2009, 3, 3085.
[3] Y. Lee, et al. Appl. Phys. Lett. 2003, 82, 3779.
[4] Y. Sun, et al. J. Appl. Phys. 2006, 100, 073106.
[5] I. Bergenti, et al. Org. Electron. 2004, 5, 309.
[6] Y. Zhang, et al. Chem. Phys. Lett. 2010, 495, 161.

Backbone fluorination of conjugated molecules and polymers has attracted much attention in the recent years due to its effect on polymer energy levels and solid-state packing.^1,2 Fluorinated polymers are often more prone to aggregation, and intra- and intermolecular interactions have been suggested to play a key role in these effects. Surprisingly little work has been made to investigate the fluorination of the most common π-conjugated polymer P3HT, with only Roncali and coworkers exploring the electropolymerisation of a singly fluorinated terthiophene unit.³
In this work, we route the synthesis of poly(3-octyl-4-fluorothiophene) and it&’s branched side-chain analogue poly(3-(2-ethylhexyl)-4-fluorothiophene), and compare various properties with their non-fluorinated counterparts. We find a significant increase in melting and crystallisation temperatures, and attribute this to an intramolecular planarising S-F interaction in combination with a larger barrier to rotation about the thiophene-thiophene bond, as suggested by Raman spectroscopy and DFT calculations.
1. Y. Sakamoto, S. Komatsu, and T. Suzuki, J. Am. Chem. Soc., 2001, 123, 4643-4.
2. Y. Wang, S. R. Parkin, J. Gierschner, and M. D. Watson, Org. Lett., 2008, 10, 3307-10.
3. F. Gohier, P. Frere, and P. D. J. Roncali, J. Org. Chem., 2013, 1-7.

The multiple potentials offered by organic electronics have been well described in terms of chemically tailored properties, mechanical flexibility, earth abundance of the basic materials, solution processability and low cost. Formulated into functional inks, organic semiconductors can be processed with printing technologies, thus allowing scalable device fabrication across large-area and onto flexible substrates, opening a complete new pathway for organic electronics and its integration into circuits.^[1] Organic electronics therefore enables a manifold of novel and alternative applications in portable, mechanically robust and light-weight devices. In particular, in the case of organic photodetectors, all-plastic short range data communication, plastic digital and conformable imagers, positions and security sensors, and interactive surfaces become possible. To date the examples of all-printed photoresponsive devices are very scarce,^[2] and limited to the case of blends comprising polymer donors. Small molecules are also appealing, as they are not affected by polydispersity and are characterized by an easier synthesis, which also fulfills the purpose of a better scalability of organic electronics. While inkjet printed blends of semiconducting polymer have already been shown to be highly reproducible,^[3] additional issues have to be solved when dealing with the printing of small molecules-based blends. The major problem arises from the strong tendency of such small molecules to crystallize limiting the uniformity and the reproducibility of the printed film. However, in donor acceptor blends a proper phase intermixing is required in order to improve charge separation.
In this work we make use of a recently synthesized low band-gap small molecule^[4] and we achieve a reproducible printing by introducing a semiconducting polymer to obtain a ternary blend. This strategy allows to greatly reducing issues related to poor printability and low device performances. We demonstrate an all-organic and fully-printed photodiode, which thanks to semitransparent contacts enables double-side signal detection: light detection is indeed showed to occur both when light is incoming from the top and from the bottom side of the photodiode, with comparable efficiencies.
[1] F. C. Krebs, Solar Energy Materials and Solar Cells 2009, 93, 394
[2] G. Azzellino, A. Grimoldi, M. Binda, M. Caironi, D. Natali, M. Sampietro, Advanced Materials 2013, 25, 6829
[3] A. Teichler, J. Perelaer, U. S. Schubert, Journal of Materials Chemistry C 2013, 1, 1910
[4] T. S. van der Poll, J. A. Love, T. Q. Nguyen, G. C. Bazan, Advanced Materials 2012, 24, 3646

Azulene, a nonalternant hydrocarbon, has attracted much attention lately because of its unusual properties, typified by a large dipole moment and long-wavelength absorption. The azulene backbone can be used to trigger the construction of materials with narrow energy gaps. In this study, we present the terazulene structural isomers 2,6prime;:2prime;,6Prime;-terazulene (TAz1) and 2,2prime;:6prime;,2Prime;-terazulene (TAz2), wherein three azulene units are linearly connected at the 2,6-position. These molecules are the first example of linear oligoazulene and are expected to enhance the unique properties of the azulene unit, including the internal dipole moment and electronic state. We report the synthesis and properties of these terazulene isomers (TAz1 and TAz2) and their application to organic field-effect transistors (OFETs).
TAz1 and TAz2 were prepared in four steps from 2-chloroazulene-6-boronic acid pinacol ester derivative via Suzuki-Miyaura cross-coupling reaction. This starting material is a novel compound and a valuable synthetic intermediate for selective substitution at the 2- and 6- positions in the azulene moiety. Top-contact OFETs were fabricated by vacuum deposition of TAz1 or TAz2 with an active layer thickness of 60 nm on an ODTS (octadecyl trichlorosilane)-treated Si/SiO₂ substrate.
TAz1 showed clear n-channel operation with an electron mobility of up to 0.29 cm² V^-1 s^-1. The LUMO of TAz1 is well distributed over the entire molecule, whereas the HOMO is localized at one end of the azulene unit. This finding indicates that the molecule is disadvantageous for hole carrier transport and advantageous for n-type-specific transport. In contrast, TAz2 exhibited ambipolar transistor performance (µ_hole = 4.9 × 10^-2 cm² V^-1 s^-1 and µ_electron = 2.1 × 10^-2 cm² V^-1 s^-1), where the HOMO, as well as the LUMO, is distributed over the entire molecule. The asymmetry between HOMO and LUMO in terazulene is derived from the asymmetric character of HOMO and LUMO in azulene, which is a nonalternant, non-benzenoid aromatic hydrocarbon. This concept, molecular orbital distribution control, offers a new strategy for controlling the polarity of organic semiconductors.

Since the discovery of semiconducting properties of organic materials^[1], numerous pi-conjugated structures have been developed to improve performances, stability and processability of devices. Especially, the last years, an increasing effort of synthesis have been carried out materials for n-type OFET^[2]. On other hand, the TIPS-Pentacene developed by J. Anthony in 2001^[3] have led to a breakthrough in the design of small molecules by the introduction of bulky group in the center of pentacene. In this context, we focused our attention on the Triphenodioxazine^[4] - a well-known pigment in industry - with pentacyclic and quinonoid structure. In this work, the synthesized triphenodioxazine core has been substituted by cyano group to increase the electron affinity with limited steric hindrance and by TIPS bulky group to generate a “brick layer” arrangement. Then, the effect of position and number of cyano-substitution on electronic properties, intermolecular arrangement, film morphology and devices performances has been studied. Firstly, weak N-H interaction has been observed and should help the 2-D growing of crystal. Secondly, one organic material has led to solution processable air-stable n-type OFET with high performances. In detail, the mobility for sublimated process in Bottom gate top contact architecture reached 0.11 cm².V^-1.s^-1 (I_on/I_off 10⁶, hysteresis = 0 V) under nitrogen and 0,11 cm².V^-1.s^-1 under air condition without any temperature annealing while the liquid processed OFET led to 0,05 cm².V^-1.s^-1. After air-storage during 1 month in the dark and without encapsulation, the mobility decreased about 25 % and V_t increased 3 V revealing the robustness of this material.
References
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[2] Y. Zhao, Y. Guo, Y. Liu, Adv. Mater.2013, 25, 5372.
[3] J. E. Anthony, J. S. Brooks, D. L. Eaton, S. R. Parkin, J. Am. Chem. Soc.2001, 123, 9482.
[4] Y. Nicolas, F. Castet, M. Devynck, P. Tardy, L. Hirsch, C. Labrugère, H. Allouchi, T. Toupance, Org. Electron.2012, 13, 1392.

Over the past decade reticular chemistry has emerged as a fundamentally new approach in organic materials synthesis. Its application was first demonstrated for the synthesis of gas storage materials, where small molecules were covalently linked through solution processing to form porous 3D networks. The resulting materials, referred to as covalent organic frameworks, are powders that exhibit unique materials properties including high porosity, periodicity, and thermal stability. Scientists at the Xerox Research Centre of Canada have developed chemistry and processing conditions which enable production of covalently-linked macroscopic films that are patterned at the molecular level. We refer to the resulting materials as structured organic films (SOFs).
During this talk the SOF materials platform will be introduced and the development of building block linking chemistries to produce new semiconducting SOF compositions will be discussed. Currently, the linking of SOF building blocks by imine bonds is being explored to take advantage of (a) no persistent by-products formed during the reaction between aldehydes and amines and (b) the numerous amine and aldehyde/ketone functionalized building blocks available. The chemistry and processing used to generate this new class of fully-conjugated imine-linked SOFs as well as their resulting physical and electronic properties and device performance will be presented.

This presentation will survey recent examples from our lab in which we have created low band gap materials based on cyclopentafused-polycyclic aromatic hydrocarbons (CP-PAHs). We have prepared both small-molecule and polymeric variants that incorporate CP-PAH scaffolds as conjugated species. Our synthetic strategy utilizes scalable processes to create halogenated CP-PAHs that can participate in a variety of palladium-catalyzed coupling chemistries. This strategy allows easy modulation of the frontier orbital energies, and thus provides access to tunable materials. I will describe the synthesis and properties of materials based on 2,7-dibromocyclopenta[hi]-aceanthrylene, 2,8-dibromodicyclopenta[de,mn]tetracene, 4,9-dibromo-1,2,6,7-tetraphenyldicyclopenta[cd,jk]pyrene, and 6,9-dibromo-1,2-diphenylcyclopenta[cd]perylene. Owing to their CP-PAH nature, these compounds have reduced lowest occupied molecular orbitals (LUMOs) and behave as electron accepting materials.

Poly(benzo[1,2-b:4,5-b&’]dithiophene-thieno[3,4-c]pyrrole-4,6-dione) (PBDTTPD) is one of the most efficient polymer donors in BHJ solar cells with fullerene acceptors, such as phenyl-C61/71-butyric acid methyl ester (PCBM).[1-4] While several PBDTTPD derivatives and other wide-bandgap analogs can be used to achieve conventional BHJ devices (single-cell) with high open-circuit voltages >0.9 V, high fill-factors of ca. 70%, and power conversion efficiencies (PCEs) >7%, our recent developments indicate that the pattern of side-chain substituents, including alkoxy (e.g. linear vs. branched),[3] alkyloyl,[5] and ring substituents,[6] critically impact polymer performance in BHJ thin films with PCBM. To date, however, the effects of the polymer side-chain pattern on (i) polymer self-assembly and nanostructural order, (ii) the development of the phase-separated morphologies (nano- and meso-scale), and (iii) the efficiency of the polymer-fullerene interface in the BHJ thin films, are only partially understood. Examining a manifold of PBDTTPD and wide-bandgap analogs via various structural (X-ray, SS-NMR), imaging (TEM, tomography) and spectroscopic techniques,[7-9] we show that the polymer side-chain pattern is a determining parameter in the interplay between polymer donor and PCBM acceptor in BHJ solar cells. It is critical to identify the factors that limit BHJ solar cell efficiency in order to continue improving device PCEs beyond 8-9% (single-cell).
[1] P. M. Beaujuge, and J. M. J. Fréchet, JACS 2011, 133, 20009.
[2] C. Piliego, T. W. Holcombe, J. D. Douglas, C. H. Woo, P. M. Beaujuge, and J. M. J. Fréchet, JACS 2010, 132, 7595.
[3] C. Cabanetos, A. El Labban, J. A. Bartelt, J. D. Douglas, W. R. Mateker, J. M. J. Fréchet, M. D. McGehee, and P. M. Beaujuge, JACS 2013, 135, 4656.
[4] J. A. Bartelt, J. D. Douglas, W. R. Mateker, A. El Labban, C. J. Tassone, M. F. Toney, J. M. J. Fréchet, P. M. Beaujuge, and M. D. McGehee, Adv. Energy Mater. 2014, Online.
[5] J. Warnan, C. Cabanetos, R. Bude, A. El Labban, Liang Li, and P. M. Beaujuge, Chem. Mater. 2014, 26, 2829.
[6] J. Warnan, A. El Labban, C. Cabanetos, E. Hoke, C. Risko, J-L. Brédas, M. D. McGehee, and P. M. Beaujuge, Chem. Mater. 2014, 26, 2299.
[7] J. Warnan, C. Cabanetos, A. El Labban, M. R. Hansen, C. Tassone, M. F. Toney, and P. M. Beaujuge, Adv. Mater. 2014, Online.
[8] K. R. Graham, C. Cabanetos, J. P. Jahnke, M. N. Idso, A. El Labban, G. O. Ngongang Ndjawa, B. F. Chmelka, A. Amassian, P. M. Beaujuge, M. D. McGehee, 2014, Submitted.
[9] C. Dyer-Smith, I. A. Howard, C. Cabanetos, A. El Labban, P. M. Beaujuge, and F. Laquai, 2014, Submitted.

The evolution of organic electronics is now poised to enter the commercial phase, with the recent market introduction of the first prototypes based on organic transistors fabricated from solution. Understanding the impact of both the organic semiconductor design and processing conditions, on both molecular conformation and thin film microstructure has been demonstrated to be essential in achieving the required optical and electrical properties to enable these devices. Polymeric semiconductors offer an attractive combination in terms of appropriate solution rheology for printing processes, mechanical flexibility for rollable processing and applications, but their optical and electrical performance requires further improvement in order to fulfil their potential. Organic solar cell efficiencies are currently increasing rapidly based on organic bulk heterojunction devices fabricated from solution. Central to these device efficiency improvements are the development of new photoactive semiconducting donor and acceptor materials, designed at the molecular level to optimise both absorption of the long wavelength region of the solar spectrum and generation of high cell voltages. This presentation will examine some of the key design strategies to control the molecular orbital energy levels and microstructure of donor polymer semiconductors and illustrate with examples and their characterisation. Specifically, the systematic reduction of the bandgap in a series of bridged ladder type indenofluorene copolymers, in combination with the progressive lowering of the HOMO energy level will be shown. Analogues of these polymers also exhibit high charge carrier mobilities, and we will present transistor data.

Controlling the optical properties of conjugated materials, especially their band gaps, is critical to nearly all of applications of these materials. The most prevalent strategy for altering band gaps of these materials involves changes to the structures of their conjugated backbones, while side-chains are generally reserved for imparting solubility. Using a series of conjugated oligo(phenylene-ethynylenes) with alternating donor (alkoxyphenyl) and acceptor (terephthalate) units, we demonstrate new examples of how altering side chains can that are not formally part of the conjugated backbone can have significant effects on bandgaps of these materials. Cofacial interactions between perfluorinated side-chains and main-chain rings in three-ring conjugated oligomers cause twisting of the main chains and yields a significant hypsochromic shift relative to the same compound in solution, while phenol-substituted oligomers show a bathochromic shift due to intramolecular hydrogen bonding. In addition, structurally related conjugated oligomers with long alkyl chains show thermally reversible piezochromism. These results indicate that synthetically simple side-chain substitutions of non-conjugated groups may be useful in rational design of the optoelectronic properties of conjugated materials in the solid state.

New emitters that can harvest both singlet and triplet excited states to give 100% internal conversion of charge into light, are emerging as viable alternatives to Ir based phosphors in organic light emitting diodes (OLEDs). Molecules that have a charge transfer (CT) excited state can achieve high efficiency through the mechanism of thermally activated delayed fluorescence (TADF).[1] We will present our latest results on the synthesis, photophysical properties and devices using new donor-acceptor (D-A) CT systems, e.g. small molecules based on carbazole donor and dibenzothiophene-S,S-dioxide acceptor units.[2,3] We will show that a D-A charge transfer molecule in the solid state, can emit not only via an intramolecular charge transfer (ICT) excited state, but also from exciplex states, formed between the molecule and the host material. For example, OLEDs based on a D-A-D molecule in a host TAPC achieve over 14% external electroluminescence yield and show nearly 100% efficient triplet harvesting. In these devices it is established that the triplet states are harvested via TADF, but more interestingly these results are found to be independent of whether the emitter is the ICT state or the D-A-D/host exciplex. [4]
[1] Goushi, K.; Yoshida, K.; Sato, K.; Adachi, C. Nature Photonics, 2012, 6, 253; Zhang, Q.; Li, J.; Shizu, K.; Huang, S.; Hirata, S.; Miyazaki, H.; Adachi, C. J. Am. Chem. Soc. 2012, 134, 14706.
[2] Moss, K. C.; Bourdakos, K. N.; Bhalla, V.; Kamtekar, K. T.; Bryce, M. R.; Fox, M. A.; Vaughan, H. L.; Dias, F. B.; Monkman, A. P. J. Org. Chem. 2010, 75, 6771.
[3] Dias, F. B.; Bourdakos, K. N.; Jankus, V.; Moss, K. C.; Kamtekar, K. T.; Bhalla, V.; Santos, J.; Bryce, M. R.; Monkman, A. P. Adv. Mater. 2013, 25, 3707.
[4] Jankus, V.; Data, P.; Graves, D.; McGuinness, C.; Santos, J.; Bryce, M. R.; Dias, F. B.; Monkman, A. P. Adv. Funct. Mater.2014, manuscript accepted (adfm.201400948).

Understanding of charge injection in organic semiconductor devices is vital for understanding and optimizing device performance. We derive an analytical equation to describe injection limited currents in organic semiconductor devices. The derivation is based on a recently developed model for the diffusion current in organic diodes. Using this formalism the diffusion current across an injection barrier can be calculated by incorporating an expression to account for the probability of creating a low energy injection pathway in the disordered semiconductor. We show the applicability of this formalism on a benchmark conjugated polymer and explain the luminescent efficiency of organic light-emitting diodes with a non-Ohmic electron contact.

Liquid electrolytes, including ionic liquids and ion gels, are being used as gate dielectric materials to yield very high surface charge densities at low gate biases. The high capacitance results from the formation of an electric double layer at the surface of the material of interest from the mobile charges in the liquid electrolyte. The use of liquid electrolyte dielectrics has allowed the exploration of many interesting devices and physical processes, such as organic field-effect transistors with low turn-on voltage, insulator-to-metal transition in organic materials, and insulator-to-superconductor transition in oxides. Here we use charge-modulation Fourier transform infrared (CM-FTIR) spectroscopy to quantitatively probe charge carrier injection at ion-gel/organic semiconductor interfaces. We show the intrinsic doping of organic semiconductors with charge densities as high as 10¹³ cm^-2 upon the formation their interface with the ion-gel in the absence of gate bias. The application of gate-bias simply increases or reduces surface charge density from this base value. We attribute the intrinsic doping to a thermodynamic effect, resulting from the interfacial stabilization of majority carriers in the semiconductor by mobile counter-ions in the electrolyte. This intrinsic doping mechanism is of general significance to liquid electrolyte based devices.

Understanding the nature of charge trapping in conjugated polymers has been a central issue because traps play a critical role in determining the performance of devices. Although charge trapping is taken for granted in modeling the characteristics of organic semiconductor devices, very few techniques actually exist to spectroscopically pinpoint trap states. In this study, we demonstrate that single-molecule spectroscopy can track the formation of charge and exciton traps in conjugated polymers in real-time, and furthermore can probe the energy level of traps directly.¹ The fluorescent keto-defect in polyfluorene was used as a model system because of its emission property (i.e. spectrum and lifetime) that is distinct from that of pristine polyfluorene². The simultaneous measurement of emission spectrum and lifetime from individual chains enabled us to observe the sequential formation of one defect after another along a single chain. Furthermore, even within a single chain, the difference in emission properties between defects was observed. Spectroscopic characterization of individual defects revealed an unexpected energetic spread of these monomolecular sites, spanning almost 1 eV. The giant distribution of trap levels presumably arises due to highly local environmental differences between individual defects, related to chain conformation and the resulting broad distribution of chromophore lengths. The energetic distribution obtained provides crucial information for quantitative modeling of macroscopic devices.
1. Adachi, T.; Vogelsang, J.; Lupton, J. M. J. Phys. Chem. Lett. 2014, 5, (3), 573-577.
2. Adachi, T.; Vogelsang, J.; Lupton, J. M. J. Phys. Chem. Lett. 2014, 5, 2165-2170.

The nexus between film microstructure and electronic properties is of paramount importance in order develop quantitative models describing charge transport in molecular solids where molecules are bound by weak van der Waals interactions. This is of particular importance in the case of recently developed polymer semiconductors showing charge mobility in excess of 1 and 10 cm²/Vs for electrons and holes, respectively, which forces to critically reconsider part of previous assumptions on charge transport in polymer films. In this context, while pure microstructural investigations, such as those based on X-rays, electron microscopy, or polarized optical probes, provide necessary information for the rationalization of transport in macromolecular solids, a general model predicting how charge accommodates within structural maps is not yet available. Therefore, techniques capable of directly monitoring how charge is distributed when injected into a polymer film and how it correlates with structural domains can help fill this gap. With the support of density functional theory calculations, we show that polarized charge modulation microscopy (p-CMM)^1,2 can unambiguously and selectively map the orientational order of the only conjugated segments that are probed by mobile charge in the few nanometer thick accumulation layer of a high-mobility polymer-based field-effect transistor. In particular we test this technique on different polymer films where the microstructure and charge mobility is controlled by the amount of pre-aggregation of chains in the solution.³ Depending on the specific solvent-induced microstructure, p-CMM can image charge-probed domains that extend from submicrometer to tens of micrometers size, with markedly different degrees of alignment. Wider and more ordered p-CMM domains are associated with improved carrier mobility. This observation evidences the unprecedented opportunity to correlate, directly in a working device, electronic properties with structural information on those conjugated segments involved in charge transport at the buried semiconductor-dielectric interface of a field-effect device. Moreover these findings help to underline the critical role of film interconnectivity and long-range orientational ordering on the transport properties of a polymer where transport involves highly localizes, intra-molecular polaronic species.⁴
[1] C. Sciascia, N. Martino, T. Schuettfort, B. Watts, G. Grancini, M. R. Antognazza, M. Zavelani-Rossi, C. R. McNeill, M. Caironi, Adv.Mater. 23 (2011) 5086
[2] N. Martino, D. Fazzi, C. Sciascia, A. Luzio, M. R. Antognazza, M. Caironi, ACS Nano, 2014, DOI: 10.1021/nn5011182
[3] A. Luzio, L. Criante, V. D'Innocenzo, M. Caironi Scientific Reports 3 (2013) 3425[4] V. D&’Innocenzo, A. Luzio, A. Petrozza, D. Fazzi, M. Caironi, Adv. Funct. Mater., 2014, DOI 10.1002/adfm.201400394
[4] V. D&’Innocenzo, A. Luzio, A. Petrozza, D. Fazzi, M. Caironi, Adv. Funct. Mater., 2014, DOI 10.1002/adfm.201400394

Organic semiconductors and their bulk heterojunctions (BHJs) have attracted considerable attention due to their potential applications in low-cost (opto)electronic devices such as organic solar cells. Since many of these applications rely on efficient charge carrier photogeneration, it is important to understand mechanisms of photogeneration depending on the molecular packing in pristine materials and on the relative energies of the donor (D) and acceptor (A) molecules, D/A packing at the D/A interface, and film structure and morphology in BHJs.
We present comprehensive studies of charge carrier photogeneration in pristine organic semiconductors and several small-molecule D/A blends. For these, we performed measurements of time-resolved photocurrent, photoluminescence (PL), PL imaging, and X-ray diffraction, as well as numerical simulations of the photocurrent. For our studies of pristine materials, we chose functionalized anthradithiophene (ADT) derivatives with different π-stacking motifs and investigated optoelectronic properties of single crystals and spin-cast thin films. For studies of D/A BHJs, we used spin-cast thin films of a high-mobility fluorinated ADT derivative as the donor and several acceptor molecules, including functionalized pentacene (Pn), indenofluorene (IF), and fullerene (PCBM) derivatives. These were selected to obtain various competitive paths for photoexcitation relaxation, which include ultrafast charge carrier photogeneration, formation of emissive (with Pn acceptors) or non-emissive (with IF acceptors) charge transfer (CT) excitons, and formation of dark and emissive donor excitons which may or may not contribute to charge photogeneration. Fits of experimentally measured transient photocurrents with numerically simulated data allowed us to quantify efficiencies of these paths and their contributions to charge photogeneration.
In all materials studied, we observed fast charge carrier photogeneration and charge carrier and exciton dynamics dependent upon molecular packing. Up to four-fold enhancement in ultrafast charge carrier generation was observed in D/A blends with PCBM added at concentrations of 7-10 wt%, as compared to that in pristine donor films. Further addition of PCBM or addition of other acceptor molecules inhibited this charge photogeneration channel. These results were directly correlated with the donor crystallinity, which was considerably enhanced in best-performing blends. Evidence of PCBM crystallization was also observed in blends with 7 and 10 wt% of PCBM. In all D/A blends, the contribution of CT states to charge photogeneration was relatively small due to CT excitons favoring emission or fast non-radiative relaxation, depending on the acceptor. Temperature dependence of charge photogeneration in these materials was also studied and will be presented.

In polymeric semiconductors, charge carriers are polarons, which means that the excess charge deforms the molecular structure of the polymer chain that hosts it. This results in distinctive signatures in the vibrational modes of the polymer. Here, we probe polaron photogeneration dynamics at polymer:fullerene heterojunctions by monitoring its time-resolved resonance-Raman spectrum following ultrafast photoexcitation. We conclude that polarons emerge within 300 fs. Surprisingly, further structural evolution on le; 50-ps timescales is modest, indicating that the polymer conformation hosting nascent polarons is not significantly different from that near equilibrium. We interpret this as suggestive that charges are free from their mutual Coulomb potential because we would expect rich vibrational dynamics associated with charge-pair relaxation. We address current debates on the photocarrier generation mechanism at molecular heterojunctions, and our work is, to our knowledge, the first direct probe of molecular conformation dynamics during this fundamentally important process in these materials.

One major advantage of organic solar cells is the possibility for large area production at low cost. This can be achieved mainly by avoiding other thin film technologies based on cleanrooms and vacuum chambers. Depositing organic semiconductors from solution at low temperature makes the use of direct printing methods feasible. One promising future application is the integration of OPV modules into windows, which is attractive for both the glass and the OPV industry. In common production methods for OPV modules mostly roll-to-roll “printing” on PET foils is used. For integration of OPV modules in insulating glass windows they are attached under heat and pressure to the glass panes and laminated afterwards. Alternatively inkjet-printing is used to avoid this consecutive costly process and to allow accurate depositing on the substrate. Although inkjet-printing is widely used for printing organic electronics, relatively few reports are available for application to OPVs. Contrary to polymer light emitting diode displays large areas have to be coated homogenously for photovoltaic applications. Solvent based inks are required to be slow drying to prevent nozzle clogging and be non-destructive for the printhead. This excludes the use of solvents like o-Dichlorobenzene, Tetraline and Mestitylene. We present here a study on different ink formulations and resulting solar cell characterization on an inverted structure: As solvents several halogen free organic liquids and their mixtures were investigated for regioregular poly(3-hexylthiophene): phenyl-C-61-butyric acid methyl ester (P3HT: PCBM) mixtures. In order to achieve suitable optical density for the active layer, inks with different concentrations of the active material in the preferred solvents were prepared. With the highest tested concentration two printed layers of photoactive material were sufficient for the desired optical parameters. For solar cell characterization photoactive layers were printed on an ITO/ZnO substrate in ambient atmosphere and dried at 60 °C in the dark. The printed area was 2.75 cm² yielding in four solar cells per printed area. The inverted solar cell structures were finalized by deposition of PEDOT as electron blocking layer and evaporation of 300 nm of Ag as top electrode. The OPV cells showed a high V_oc of 0.61 V, a filling factor of 50 to 60% and J_SC of 7 - 7.5 mA/cm². Efficiencies of 2.5 % were achieved
The authors acknowledge funding from the European Community&’s 7th framework program under grant agreement No. 314578 MEM4WIN

Understanding and ultimately controlling exciton dynamics is essential to quantifying the nature of optoelectronic organic thin films, and in optimizing the characteristics of devices in which they are employed. Here, we study the temperature dependence of triplet and singlet exciton dynamics in the archetype small molecule fluorescent guest-host system, tris(8-hydroxyquinolinato) aluminium (Alq₃) doped with 4-(dicyanomethylene)-2-methyl-6-julolidyl-9-enyl-4H-pyran (DCM2). We develop a comprehensive model of the exciton dynamics, and use it to fit the transient photoluminescence under different pulsed optical pumping in the temperature range of 80 K< T< 295 K. Our work shows that triplet decay has a significantly different temperature dependence than that of singlet. From 295K to 80K, the triplet-triplet annihilation (TTA) rate decreases by two orders of magnitude, whereas the singlet-triplet annihilation rate decreases by <50% primarily as a result of the different energy transfer mechanisms of singlets and triplets. The temperature dependence of the TTA rate reveals two regimes separated by a transition at 180K from Marcus to Miller-Abrahams transfer. This work deepens our understanding of exciton dynamics and energy transfer in small molecule organic materials.

It is well established that the performance of organic solar cells strongly depends on the morphology of the photoactive layer that is formed during deposition. Although numerous studies have described the final morphology formed, the important question how processing conditions influence performance remains to be answered. Particular intriguing in this respect is the role of co-solvents that are being used in virtually all high-efficiency organic solar cells. Apart from some general experimental guidelines, the precise role of these co-solvents is subject of intense discussion. An important reason is the lack of studies that follow morphology formation in time. We have resolved morphology formation in time by combining three optical measurements. We investigate layer thickness, phase separation, and polymer aggregation during solvent evaporation under realistic processing conditions with subsecond time resolution. We find that the role of the co-solvent is to induce polymer aggregation prior to liquid-liquid phase separation. This result sheds new light on the processing conditions that give high efficient organic solar cells.

Organic devices have been demonstrated to sense variable physical parameters such as light, pressure and temperature. For temperature sensing the sensitivity is related to the conductivity variation of organic thermistor. Due to the low intrinsic conductivity activation energy of bulk organic semiconductor, thermally induced charges contributed very little to the electrical conductivity enhancement of organic thermistor made by pentacene or dinaphtho[2,3-b:2prime;,3prime;-f]thieno[3,2-b]thiophene (DNTT) thin films and thus resulting in poor sensitivity of temperature sensor. For pentacene, the bulk conductivity activation energy is 180 meV; and for DNTT, the temperature induced conductivity change is extremely weak and it is difficult to determine the activation energy due to the measurement limit. In this work we introduced different metal nanoparticles into DNTT and pentacene by both co-evaporation and layer-by-layer deposition methods to form a hybrid organic/metal thermistor. By inserting the up to 20% volume ratio Ag NPs into DNTT matrix, the activation energy was enhanced from temperature insensitive pure DNTT to 420 meV and the conductivity variation increases two orders of magnitude during the measured temperature range from 293K to 373K. When further increasing the Ag volume ratio, individual NPs are tend to connect together to form a conductive network that overall conductivity could enhanced about 1000 times and the high temperature sensitivity will be lost. Based on the hybrid organic/metal thermistor, an integrated device array that can operated at low voltage (5V) is fabricated on flexible substrate, each of device consist of one thermsitor and one low voltage oragnic thin film transistor for addressing and signal amplification. The mechanism of the increasing activation is investigated and attributed to the extra traps states induced by metal NPs in band tail of highest occupied molecular orbital (HOMO) of semiconductor. The low voltage flexible thermal sensor array is suitable for portable electronic devices and potentially scale up for electronic skin applications. Temperature detection for health monitoring or as a surgery tool can be achieved by the current device.

A novel technique based upon injection-charge extraction by linearly increasing voltage (i-CELIV) in a metal-insulator-semiconductor (MIS) diode structure was used to study the charge transport in organic solar cell. This technique can measure both hole and electron separately, by using different device configuration. By contrast, normal CELIV only measures the faster charge carrier in organic semiconductor without the capability of differentiating the type of charge carrier. Moreover, normal CELIV often relies upon the light excitation to generate free charge carriers due to low free carrier concentration of organic semiconductors while MIS-CELIV allows for the measurement of charge carrier in the dark. In our work, this technique was modified with a thick LiF of ~100 nm as the blocking layer in an organic photovoltaic cell, which can effectively block a certain type of charge carriers. As a result, electron or hole mobility can be measured from electron only or hole only MIS-diode. By applying different offsets in the dark MIS-CELIV, the trap information can be clearly revealed. Furthermore, photo-generated carriers under light illumination and extracted carriers in the dark can be completely distinguished by comparing the area of current density vs time curve between the dark and light MIS-CELIV. Through the studies of P3HT and P3HT:PCBM blend, as proof-of-concept, we found that the hole mobility of P3HT is increased by one order of magnitude under light illumination than that in the dark, while the hole mobility of P3HT:PCBM blend is only twice as that in the dark. This indicates that the hole and electron balance is achieved in the P3HT:PCBM blend, leading to the high efficiency in the photovoltaic cell.

Conducting polymers have been widely used in batteries, catalyzer, supercapacitors, electrochromic and electroluminescent devices and also for corrosion control. A number of researches about conducting polymers have been increased during two decades. Polyaniline (Pani) is one of the most potential conducting polymers due to its easy synthesis, environmental stability, high conductivity and unique redox properties. Also it can be easily electrochemically synthesized in aqueous solution, however it&’s necessary to realize a systematic study that show the influence of the substrate type and of the concentration in the kinetics of growth.
In this work we present a systematic electrochemical study of the growth of Pani Films on ITO and stailess steel 304 substrates by using different concentration of aniline (10^-1M, 10^-2M, 10^-3M) in H₂SO₄ and HCl solutions (1M). Pani films were electrodeposited by using cyclic voltammetry (CV) at maximum limit anodic potentials (E_l) of 0.9V and 1.0V vs. SCE, while the lower limited potential was -0.2V vs. SCE. Chronoamperometry (CA) was carried out at two applied potentials (E_ap): 0.9V and 1.0 V vs. SCE, while chronopotenciometry (CP) was carried out varying the current applied between 10^-6 and 10^-2A. The cyclic voltammograms showed anodic/cathodic peaks associated at the different stages of growth and its correlation with oxidation states de Pani, the potential (E_p) and currents (I_P) of the peak showed small differences as a function of monomer concentration on the different substrates. Moreover the electrochemical growth of Pani on the stainless steel electrodes showed a strongly influence in the anodic and cathodic peaks, at potential more positive due to the hydrolysis reaction (1.0 V vs SCE) and it also influenced in the topography of Pani films according to AFM results.
Acknowledgment
This research was supported by Promep/103.5/11/6714, PAPIIT-DGAPA (IB101512-RR181512), CONACyT-México (CB176450) and PIFI.

Chemical doping of organic semiconducting films has become important for manipulating key device parameters, e.g. lowering charge injection barriers, increasing conductivity, and controlling the density of active trap states in the semiconductor material [1]. We demonstrate here the finely controlled enhancement of the electrical conductivity of solution-processed thin polymer films, by varying the concentration of electron donating molecules (n-dopants) within the regioregular conjugated polymer P(NDI₂OD-T₂). P(NDI₂OD-T₂) is an electron-transporting material, which exhibits air stability and high mobility in organic thin-film transistors [2]. The addition of the strongly reducing air-stable organometallic dimer, RhCpCp*, allows for the controlled filling and de-activation of electron trap states distributed energetically below the lowest unoccupied molecular orbital (LUMO) [3]. By utilizing variable temperature current-voltage measurements, we observe an increase in the bulk conductivity of the film by six orders of magnitude with doping concentrations ranging from 7×10^-5 to 3×10^-2 molar ratio (MR). Ultralow doping concentrations are necessary to observe the transition from trap filling to ohmic transport. Analysis of the current density versus electric field in P(NDI₂OD-T₂) films yields a clear trend from trap-charge limited regime at lower doping concentrations to ohmic regime at higher doping concentrations. The transition between these two regimes allows for a quantitative estimation of the density of trap states in undoped P(NDI₂OD-T₂).

Singlet exciton fission is a process that occurs in certain organic materials whereby one singlet exciton splits into two independent triplet excitons. In photovoltaic devices these two triplet excitons can each generate an electron, producing quantum yields per photon of >100%. Here, we study the photoexcitation dynamics in antradithiophene derivatives using ultrafast photoinduced absorption spectroscopy. At room temperature, we observe the decay of singlet excitons is associated with the growth of triplet excitons within 50 picoseconds time-scale. We attribute this effect to the formation of triplet excitons via the singlet exciton fission to generate two triplet excitons from a single absorbed photon, which reduces the photoluminescence quantum yield. More importantly, by modifying the side-chains of the molecules and hence the morphology of the neat film, singlet exciton fission can be slowed-down to nanosecond time-scale and eventually been turned-off. This work lays the foundation for ways to tailor the multicarrier generation in organic semiconductor by controlling the film morphologies

A phenomenon known as the giant surface potential (GSP), where the surface potential of organic films display linear growth with increasing film thicknesses in the absence of light was first reported by Ito et al. (JAP 2002) on (8 hydroxyquinoline)aluminum(Alq₃), a prototypical fluorescent material used in OLEDs. It has been shown that the surface potential of Alq₃ has reached 28 V for a 560 nm thick film by Kelvin probe measurements in vacuum in the absence of light. Since then this phenomenon has been observed for a broad range of molecules thermally evaporated on varying substrates under similar conditions. The effect is independent of the substrate, dependent on the film thickness and decays quickly with illumination at the normal mode of the respective molecule and at elevated temperature. The spontaneous buildup of the (GSP) cannot be explained by any classical interfacial phenomena. Investigations into the cause of GSP, including the analysis of light and heat on the surface potential, are not clear.
In this study we use vibrational spectroscopy to understand the nature of the GSP buildup, where we have found a significant change in the vibrational structure of the organic material in thick films where the GSP is present as compared to thin films. The vibrational spectra of the most commonly studied light-emitting material, Alq₃, on indium tin oxide (ITO) is investigated as a function of thickness using high resolution energy electron loss spectroscopy (HREELS), Raman spectroscopy, high resolution x-ray photoelectron spectroscopy (HR-XPS), attenuated total reflectance infrared spectroscopy (ATR-IR), and density functional theory (DFT) calculations. In order to provide a holistic understanding of the GSP, the results are compared to the vibrational spectra of 1,3,5-tris(N-phenylbenzimiazole-2-yl)benzene (TPBi) on ITO, an electron transporter host material with a measured GSP of 0.07 V/nm, and bis(triphenylsilyl)-dibenzofuran (BTDF) on ITO, a typical electron-conducting host used in combination with hole-conducting deep-blue emitter with a measured GSP of 0.08V/nm. The observed spectra show significant changes with the presence of the GSP in the organic material on ITO, which can be explained in terms of different symmetries of the isomers as well as between complexes and isolated anions. Additionally, it has been found that the surface phase differs from the bulk phase, where a structured layer is evident at the interface of the organic semiconductor, and this layer shifts with increasing thickness and in the presence of the GSP. The present work has provided direct evidence that a different molecular orientation exists at the interface than in the bulk, where the GSP exists.

There is growing interest in sensors based on organic field-effect transistors (OFETs) due to their high potential for use in lightweight, flexible and wearable electronic devices. The OFET-based wearable sensors require substantially challenging properties such as high flexibility, mechanical stability, excellent electrical performance and ambient stability. Textile is one of the strong candidates for the substrates of wearable electronic devices due to its flexibility and even stretchability. Herein, we report highly flexible and mechanically stable OFETs fabricated on a textile substrate with an elastic buffer layer. We used electrospun organic semiconducting nanofibers and polydimethylsiloxane (PDMS) as the active layer and dielectric layer, respectively. Textile-based flexible OFET devices showed high-performance electrical characteristics and superior mechanical stability from the bending test with an extremely low bending radius compared with film-type substrates or PDMS-only substrates. In addition, they showed highly stable electrical performance after the bending cycle test over ~1000 cycles. Furthermore, we also investigated photo-responsive behaviors of organic semiconducting nanofibers under light irradiation with different wavelengths, with a view to applying them for wearable photosensors or photoswitches.

Spatially resolved information on transport losses and charge accumulation within operating organic electronic devices is highly desirable for optimization of device architectures. Employing scanning Kelvin probe microscopy (SKPM)¹ on device cross sections created by focused ion beam milling, we directly determine the electrical potential within OLEDs. Measurements are performed on well understood fluorescent tris-(8-hydroxyquinoline)aluminum (Alq₃) : N,Nprime;-bis(naphthalen-1-yl)-N,Nprime;-bis(phenyl)benzidine (NPB) bilayer OLEDs. By variation of bias voltage during the measurement, we find inhomogenous field distribution across the device for different operation states. Comparing different configurations of the layer stack, we conclude that widely disregarded orientation polarization of small polar molecules plays a crucial role for the efficiency of organic electronic devices.
In conjunction with current-voltage-luminance measurements, impedance spectroscopy and displacement current measurements², we present a conclusive model of potential distribution and charge accumulation within the bi-layer structure: In the regular device architecture, Alq₃ is evaporated on top of the NPB layer and permanent orientation polarization of polar Alq₃ molecules induces a negative polarization charge density at the organic-organic interface. The largest part of the applied voltage is thus sustained across the Alq₃ layer even in depletion state. With increasing forward bias, the negative polarization charge is compensated by holes that accumulate at the interface. Again the voltage drops almost mainly across the Alq₃ layer which facilitates electron injection and device turn-on.
In the inverted device structure, Alq₃ forms the bottom layer of the bi-layer device which leads to positive polarization charge at the interface to NPB and no accumulation of mobile holes is observed before device turn-on. The applied voltage is sustained almost uniformly across the entire device stack so that the electric field in the Alq₃ layer is lower than for the regular device structure. This limits electron injection and the device turn-on voltage is significantly higher than in the regular architecture, leading to dramatically reduced device efficiency. By substituting Alq₃ with propylated Alq₃ (Al(7-prq)₃) for both device architectures the sign of the interface charge is reversed³ as is the behavior of carrier accumulation and the relative device efficiency of inverted and regular structure.
We thus demonstrate the consistency of cross-sectional SKPM with established characterization methods for multi-layer OLEDs and highlight the importance of orientation polarization for the design of efficient OLED architectures.
1. Saive, R. et al., Adv. Funct. Mater.23, 5854-5860 (2013).
2. Noguchi, Y. et al., J. Appl. Phys.111, 114508 (2012).
3. Noguchi, Y. et al., Appl. Phys. Lett.102, 203306-203306-5 (2013).

Understanding electronic devices down to the atomic scale is essential for the development of novel organic molecule based nanotechnologies. Through use of scanning tunneling microscopy (STM), the self-assembly of new organic molecules can be imaged to understand their structural and electronic properties at the molecular level. 5,6,7-trithiapentacene-13-one (TTPO) is a promising organic semiconductor with potential applications in high temperature photovoltaic devices. This robust electron donor was thermally evaporated onto the close-packed stepped Au (788) surface in an ultra-high vacuum chamber. STM imaging has revealed interesting nanoscale surface structures of TTPO molecular chains anchored to the surface at low coverage as well as an ordered self-assembled monolayer on the close-packed gold surface terraces. Combining imaging with density functional theory calculations allows for classification of these self-assembled structures with particular interest being directed toward the interaction between TTPO and gold at this organic-metallic interface. Theoretical calculations have been used to probe this unique 3-D angular assembly determining a 17 degree angle between the laterally lying pentacene backbone and the gold surface, which is distinctive from previously observed pentacene and pentacene derivative assemblies on surfaces. It is the understanding of the structure of these photovoltaic heterojunctions that can allow for tailoring of interfaces with improved electrical transport and energy-conversion efficiency for future devices.

For efficient organic photovoltaic devices at open circuit voltage conditions bimolecular recombination is a major loss mechanism [1]. Understanding how recombination limits cell performance is a key interest within the organic photovoltaic community. Recombination is often probed electrically by transient photovoltage (TPV) [2] or optically by transient absorption (TA) measurements [3]. Recently we have combined these two measurements to probe both optical and electrical transient signals concurrently. A reflection geometry for the optical probe enables functional encapsulated devices to be tested under the modes of operation. Transient measurements observe the device after photoexcitation by a short laser pulse and by fitting such signals recombination kinetics can be investigated. Our experiment is tailored to probe transient signals occurring in the ns - ms range. Care is taken to account for the effect of the infra-red optical probe used in the TA measurements on recombination kinetics, in particular the ability for infra-red excitation to photo excite charge carriers and perturb TPV measurements. In this talk I will present data concerning recombination kinetics in P3HT and low-band gap polymer devices, and also address infra-red excitation in pump-probe experiments, which is currently an active topic of discussion in the field.
[1] Lakhwani, G. et al (2014) Annu. Rev. Phys. Chem. 65:557-81
[2] Elliott, L.C.C. et al (2014) Adv. Energy. Mater. 1400356
[3] Clarke, T.M. et al J. (2009) Phys. Chem. C. 113(49) 20934-41

Recent studies on the fluorocarbon polymer dielectric, Cytop, show its excellent ability to improve the environmental stability and electronic performance in organic field-effect transistors (OFETs). However, this dielectric is typically spin-coated and is therefore incompatible with large-area electronics. We report on the spray-deposition of Cytop to create a high quality insulating film using manufacturing methods suitable for the scaling up of devices. Our sprayed Cytop films exhibit excellent properties with very low leakage currents (10-100 pA/mm² at 3 MV/cm) and a high dielectric breakdown strength (> 5 MV/cm), even for films as thin as 50 nm, as observed in stacked capacitor structures. This compares to spun Cytop films that have shown higher leakage currents (~ 10 nA/mm² at 2 MV/cm) with thicknesses ~ 450 nm measured in a similar stacked structure. We evaluate the performance of this dielectric film in OFETs made with the organic semiconductor diF-TES ADT (2,8-difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene), and compare the results with those obtained with spin-coated Cytop in the same device configurations. The devices have similar properties in terms of field-effect mobilities, threshold voltages, subthreshold slopes and on/off current ratios (mu; asymp; 1 cm²V^-1s^-1, V_T asymp; -2 V, S asymp; 1 V/decade, I_on/I_off asymp; 10⁵). In addition, our sprayed films create OFETs with operating voltages of < 10 V, which is significantly lower than the typical operating voltages necessary for the spin-coated Cytop FETs. The demonstration of this sprayed Cytop, coupled with our ability to effectively spray the active organic semiconductor in OFET devices, proves to be great progress towards an all-sprayed device. The development of the sprayed dielectric layer allows for better fabrication control, reduced material usage and is immediately scalable to large-area electronic applications without compromising device performance.

The main challenge is solution processed organic semiconductors is the low charge carrier mobility associated with low degree of molecular order. Soluble small molecule semiconducting compounds developed in the recent years offer improved charge transport characteristics originating from the ability of these materials to form large crystalline domains with short pi-pi distances. The device performance of field-effect transistors with poly-crystalline channels can vary significantly depending on the uniformity of solid-state packing of the molecules. Controlling molecular order from the solution phase with a technique that is low cost, easily adoptable in large area device fabrication and compatible with a variety of organic semiconductors is one of the main challenges in organic electronics.
In this work, we demonstrate a new approach for the fabrication of organic semiconducting single crystals by a spray coating technique, and solvent-antisolvent crystallisation. Solutions of various acenes, including TIPS-pentacene, diF-TES-ADT, TES-ADT and TIPS-anthracene were spray-deposited on glass, Si/SiO₂ and plastic substrates. Spay parameters were controlled to vary crystal shape, orientation and size. The resulting isolated single crystals of TIPS-pentacene were some of the largest solution processed single crystals reported in the literature based on acene-family molecules with up to 0.65mm² area. The high degree of molecular order was verified with polarised optical microscopy, XRD, SEM and polarised Raman spectroscopy (PRS). Importantly, PRS provided direct information on the crystal quality and molecular orientation, making this technique an excellent tool for screening highly ordered organic structures. Charge transport properties of single crystals were examined by field-effect transistor technique.
In summary, we compare spray deposition of organic single-crystals with other solution-based growth methods, evaluate prospects of spray-based fabrication for large area printed electronics, and validate polarised Raman spectroscopy as an efficient characterisation tool for highly ordered organic semiconductor materials.

Thin films of poly(3-hexylthiophene) (P3HT) are promising candidates for next generation of organic based electronic devices. Chain packing and crystallinity in such films have been shown to strongly affect their electronic and optical properties (REFS). Recently, confinement of thin semiconducting films have received a lot of interest, but there is still little known about the effect of nano-scale confinement and how its geometry affects crystallization in P3HT.
Here, we present results on how the geometry of true nano-confinement (2D vs. 3D) influences crystallization and the final crystallinity in P3HT films. Thin P3HT films were either confined between two solid walls (2D), or in a three-dimensional cylinder (3D) resulting in free standing nanoscale cylinders. The films were characterized by 2D synchrotron grazing incidence X-ray diffraction (2D GIXD) and by atomic force microscopy (AFM).
The results show a strong effect of the geometry of the confinement, and a different crystalline orientation between 2D and 3D confined films. Annealing time was also shown to strongly affect crystallinity, but in a different way compared to traditionally spun and annealed P3HT films. We discuss the effect of sample preparation, temperature and annealing time on crystallinity of P3HT in thin layers under confinement, and compare it to the crystallinity in non-confined thin films.

Polymer dielectrics are promising materials where the chemical flexibility enables gate insulators with desired properties. For example, polar groups can be introduced to enhance the dielectric constant, although fluctuations in chain conformations at the semiconductor-dielectric interface can introduce energetic disorder and limit charge mobilities in thin-film transistors. Here, we demonstrate a photopatternable high-K fluoropolymer, poly(vinylidene fluoride-bromotrifluoroethylene) P(VDF-BTFE), with a dielectric constant between 8 and 11. The bromotrifluoroethylene moiety enables photo-crosslinking and stabilization of gate insulator films while also significantly enhancing the population of trans torsional conformations of the chains. Using rubrene single crystals as the active layer, charge mobilities exceeding 10 cm²/Vs are achieved in thin film transistors with cross-linked P(VDF-BTFE) gate dielectrics. We hypothesize that crosslinking reduces energetic disorder at the dielectric-semiconductor interface by suppressing segmental motion and controlling chain conformations of P(VDF-BTFE), thereby leading to approximately a three-fold enhancement in the charge mobility of rubrene thin-film transistors over devices incorporating uncross-linked dielectrics or silicon oxide.

Due to its ballistic charge transport and high electron mobility reaching 10 000 cm2middot;Vminus;1middot;sminus;1 at room temperature, graphene has been seen as one of the most exciting new materials for electronic devices. Here, we present a study of the crystallizaton of highly regioregular poly-3-hexylthiophene (P3HT) on a single layer large area graphene sheet, and we measure its charge transport in the direction perpendicular to the film. The crystalline structure of the films was characterized by 2D grazing incidence X-rays synchrotron diffraction (2D GIXD).
Conductivity measurements showed that the film deposited on graphene resulted in at least 2.5 times increase in charge transport in the vertical direction compared to a similar film deposited on silicon. This result correlates with an enhanced vertical π-π stacking and higher degree of interconnectivity between crystallites. Raman and UV-vis spectroscopy also revealed an increase in correlation length and in chain planarity on graphene. These results suggest that graphene could help improve charge transport and efficiency of organic and hybrid photovoltaics.

Organic π-conjugated molecular and polymeric thin-film electronic devices usually suffer from significant structural disorder resulting mainly from the presence of amorphous regions within the films. Understanding the disorder in such regions and its effect on charge transport can lead to more efficient devices by providing increased fundamental understanding. Here we use molecular dynamics (MD) simulations and density functional theory (DFT) calculations to evaluate the energetic disorder (the distribution of site energies) of varying clusters of [6,6]-phenyl-C₆₀-butyric acid methyl ester (PCBM)¹ and its derivatives. We compare and contrast different components to the site energies for amorphous and crystalline regions of parent and bis- and tris- adducts of PCBM. We also delineate the fundamental difference between the static and dynamic contributions to the energetic disorder.
(1) Tummala, N. R.; Mehraeen, S.; Fu, Y. T.; Risko, C.; Bredas, J. L. Adv. Funct. Mater.2013, 23, 5800.
We gratefully acknowledge the support of various parts of this work by the Deanship of Scientific Research of King Abdulaziz University under an International Collaboration Grant (Award No. D-001-433) and the Office of Naval Research (Award No. N00014-14-1-0171).

In recent decades a wide range of air stable carbon based nanomaterials such as carbon nanotubes and fullerenes have been developed for energy applications. Unfortunately the large scale implementation of these materials is frequently limited by expensive synthesis and selection procedures. Instead of synthesizing carbonaceous materials for energy applications, we report on a method of producing thin films of carbonaceous nanomaterials from abundant lignite and bitumen sources which are already produced on hundreds of megaton per year scales. Ball milling and centrifugation of powdered material is shown to produce particles in the 10&’s of nm size range, and characterization of the particles shows they preserve the original chemistry of the organic material with loss of mineral content. Electronic properties of these materials were characterized in field effect transistors.

In this study, we seek to determine if thin film deposition can be used to improve electronic/optoelectronic properties of as-synthesized organic semiconductors. For example, can a deposition technique promote polymer stacking in a specified orientation for enhanced charge transport, such as out-of-plane π-π stacking to increase vertical charge conduction in organic photovoltaic (OPV) polymers? We investigate such fundamental issues using emulsion-based, resonant infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE), in which organic thin films are deposited onto substrates by infrared laser (Er:YAG, 2.9 µm) ablation of a frozen emulsion target. The laser energy is resonant with a vibrational mode of the OH bond, which is absent from target guest materials and corresponding solvents, but present in water (emulsion host matrix).
The RIR-MAPLE emulsion target typically comprises guest organic material, primary solvent, secondary solvent, deionized (DI) water, and surfactant. The purpose of the surfactant, which has been investigated previously¹, is to stabilize the emulsion for flash freezing. The purpose of the DI water is to enrich the OH bond concentration; however, water vapor degrades organic solar cells due to adsorption onto the PEDOT:PSS layer². It is not yet clear if water vapor resulting from RIR-MAPLE has any direct impact on organic semiconductor films. Therefore, emulsion host matrix alternatives, such as alcohols, will be explored. The purpose of the low vapor pressure secondary solvent (e.g., phenol) is to provide additional OH bonds and to stabilize the target; however, there is a trade-off related to phenol content because it often leads to precipitation of the guest material. The purpose of the primary solvent is to dissolve the guest material, yet the most compatible solvent may have a high vapor pressure that destabilizes the frozen target.
Therefore, we will investigate the following emulsion target properties: choice of emulsion host matrix (water vs. alcohols), amount of secondary solvent (phenol), and choice of primary solvent (low to high vapor pressures). We will investigate these emulsion target properties for three different OPV polymers (P3HT, PCPDTBT, and F8TBT). UV-visible absorption spectroscopy will be used to characterize general film morphology. Atomic force microscopy and atomic force acoustic microscopy will be used to characterize film surface morphology and mechanical robustness, respectively. Grazing-incidence, wide angle X-ray scattering will be used to characterize polymer stacking and orientation. In this way, we will identify the extent to which RIR-MAPLE emulsion target preparation can impact organic semiconductor thin film properties.
This work is supported by the NSF Research Triangle MRSEC (DMR-1121107).
References
W. Y. Ge and A. D. Stiff-Roberts, Electron. Mat. Con., Santa Barbara, CA, June 2014.
K. Kawano, et. al., Sol. Energ. Mat. & Sol. C., 2006, 90(20), 3520-3530.

Epitaxial growth of polyaromatic molecules has been widely investigated in search of conditions for conformal growth and model systems for organic electronics. While pentacene has been a model system of choice, it&’s topoisomer picene has received hardly any attention. Picene has an armchair arrangement of benzene rings, unlike linear fusion in pentacene. This difference translates into nearly doubly-degenerate LUMO, and a tendency to form Frenkel rather than charge-transfer excitons. Moreover, potassium doping of picene was reported to produce a superconducting state with a T_c of 18K, with the working hypothesis pointing to formation of half-filled electron band derived from the LUMO states upon doping.
Here we will report on the structural and electronic properties of ultrathin films of picene grown on single-crystal metal surfaces, Ag(100), Ag(111) and Cu(110). Structural differences between picene and pentacene yet again manifest in the strikingly different morphologies of the self-assembled patterns of these compounds. Picene growth proceeds in two distinct stages: (1) formation of disordered incommensurate nearly-saturated layer with flat-lying picene molecules; (2) growth of an extraordinarily well-ordered incommensurate monolayer where the molecules are adsorbed on-edge relative to the surface (3D monolayer). Both layers nominally correspond to 1ML coverage and are stable at room temperature. The key difference of this growth process from all previous accounts of pentacene is the formation of the 3D monolayer. It exhibits strongly attractive intermolecular interactions and a tendency to flow over the steps of the underlying metal substrate, almost completely eliminating disorder at the interface. At the same time, intermolecular electronic coupling between is dramatically enhanced compared to flat-lying molecules, which may become useful in applications involving charge injection and/or separation. This structure is therefore a compelling alternative to comparatively much more defective three-dimensional monolayers of thioaromatic molecules, and an intriguing model system to understand the physics of picene. Based on first-principles calculations, the formation of the 3D monolayer is driven by sufficiently strong van-der-Waals and quadrupolar interactions within the molecular layer itself coupled with weaker interaction of the “corrugated” molecular edge with the silver surface. We therefore argue that the tendency to form such coherent monolayers could be quite general for large polyaromatic hydrocarbons with specifically tailored topology. [A portion of] This research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.
[1] S. J. Kelly, D. C. Sorescu, P. Maksymovych, “Three-dimensional self-assembly of picene on metal surfaces”, submitted (2014).

Barriers to charge transfer at electrode-semiconductor contacts are ubiquitous and limit the applicability of organics to electronic devices. Molecular or ionic doping near contacts can alleviate charge injection or extraction problems by enabling charge tunneling through contact barriers. Here, we demonstrate that polymer acids can act as p-type dopants for poly(3-hexylthiophene-2,5-diyl) (P3HT). The performance of contact-doped organic photovoltaics nearly matches the performance of devices comprised of traditional hole transport layers such as PEDOT:PSS. By varying between different pendant acid groups: sulfonic acid, triflluoromethan sulfonimide, and perfluorosulfonic acid, as well as different backbones in the polymeric dopants, we find the effectiveness of doping conjugated polymer depends on the strength of the pendant acid group with stronger acid moieties being capable of donating more free carriers to the doped system. Use of polymeric dopants allows high carrier densities of order 10²⁰ cm^-3 to be obtained in the polymer semiconductor. However, the complexity of doping extends far beyond the strength of pendant acid group alone and we find evidence that phase separation between polymeric dopants and conjugated polymers give rise to the complexity of doping. The overall doping effectiveness therefore is a delicate interplay between the strength of pendant acid groups and phase segregation between polymeric dopants and conjugated polymer.

The latest advancements in the semiconducting polymer science have evidenced the key role played by the microstructure of polymeric films in the determination of the transport properties within field effect transistor architectures, where the percolation paths of the charge carriers are sized by the micrometric lengths of the channels (few to 100 µm). Efficient, defect-free, short-range intermolecular packing is a necessary prerequisite for polymers to reach high mobility values [1]; however, recent demonstrations have highlighted the anisotropic nature of the transport of some the best performing polymers, and a boost of mobility has been obtained by adopting directional deposition techniques like rubbing [2], solution shearing [3] and directional solvent evaporation [4], which not only induce high crystallinity, but also introduce controlled, orientational order in the long-range. In fact, it is generally believed that, if molecules are aligned parallel to the applied electrical field, intramolecular charge transport occurs efficiently along the rigid backbone of the polymer chains and just few “highly coupled” intermolecular region are needed to bridge the adjacent molecules.
In this contribution we demonstrate that in the naphthalene-diimide based copolymer (P(NDI2OD-T2)) electrons field effect mobility can be controlled over two orders of magnitude by adopting solvents which introduce a different amount of chains pre-aggregation in the solution. Solution pre-aggregation was found to promote liquid-crystalline like long-range order within a nano-fibrillar thin film microstructure [5]. By deepening the self-assembling properties of P(NDI2OD-T2) we have observed that, controlling the solution concentration and the flow occurring during the film formation, an efficient ordering of the supramolecular nanostructure can be achieved.
Basing on this strategy, a neat molecular alignment on large areas was obtained using simple, scalable, low-temperature and ambient-processed directional deposition methods such as bar coating. We found that, within a macroscopically aligned morphology, electron mobility parallel to the nano-fibrils direction (i.e. to the molecular backbones, as evidenced by polarized optical absorption) overcomes the mobility perpendicular to the fiber direction of more than one order of magnitude with exceptional reproducibility, approaching 3 cm²/(Vs) at high voltages. By linking transport properties to length scales from the meso- to macro-scale this work offers a critical perspective for controlling and further improving mobility in polymer FETs.
[1] Noriega, R. et al. Nat. Mater. 12, 1038-1044 (2013).
[2] Brinkmann, M. et al. Macromol. Rapid Commun. 35, 9minus;26 (2014).
[3] Lee J. et al. J. Am. Chem. Soc. 134, 20713minus;20721 (2012).
[4] Tseng H.-R. et al. Adv. Mater. DOI: 10.1002/adma.201305084
[5] Luzio A. et al. Sci. Rep. DOI: 10.1038/srep03425

Contact resistance dominates the field-effect transistor behavior of organic field-effect transistors with high carrier mobilities and short channel lengths even in the staggered top-gate bottom-contact configuration. To investigate this problem, we have developed a self-consistent transmission line method (TLM) that enables the contact resistance R_c of organic field-effect transistors (OFETs) to be reliably extracted as a function of sourceminus;drain current and accumulation carrier density over a wide operational window. Using top-gate bottom contact (OFETs) with a high-hole-mobility diketopyrrolopyrrole (DPP) polymer (mu;_FET = 0.45 cm²V^-1s^-1) and gold source-drain electrodes modified by a proprietary solution-processed method as test models, we show that we can achieve practically ohmic contacts with contact resistivity as low as 2 Omega; cm² between the electrode and the organic semiconductor with an ionization potential (Ip) of 5.4 eV. The contact resistance begins to be dominated by the bulk space-charge resistance and channel resistance. For a sufficiently low channel resistance, current crowding does not occur over the electrodes.

State-of-the-art organic light-emitting diodes consist of a stack of layers of different small molecule-based materials. The individual layers have specific functions as hole transport layer (HTL), emissive layers (EML), or electron transport layer (ETL).Such a multilayer structure is thermally evaporated in high vacuum, which is a relatively slow and expensive process. A route towards lower cost is printing or coating these layers from solution. However, the major problem is that a deposited layer will redissolve in the solvent of the subsequent second layer. Our approach to overcome this problem is to blend an inert and insulating host matrix, which can be made insoluble via UV-crosslinking, with a functional organic semiconductor. After deposition of the blend the host matrix is cross-linked such that the whole layer becomes insoluble. In this way the electronic functionality can be separated from the processing properties. We demonstrate that for MEH-PPV as active material only 5% of host matrix is required to make the layer insoluble. This 95:5 blend has nearly the same current density as pristine MEH-PPV but is insoluble. In this way we stacked up to 5 layers of MEH-PPV without any problem. Combining the insoluble 95:5 MEH-PPV blend layer with a polyfluorene hole blocking layer led to an efficiency increase of 20% as compared to a conventional polymer light-emitting diode of pristine MEH-PPV

Excitons dominate the optoelectronic properties of disordered and low-dimensional semiconductors. In organic semiconductors, the excitons are localized and exhibit distinct spin-0 singlet and spin-1 triplet states. Triplet excitons are typically dark, but they can be the dominant excitation inside molecular optoelectronic devices as they exhibit long lifetimes, are often the lowest energy excitation in organic photovoltaic blends, and comprise three quarters of the excitons formed from free carriers. Triplets are also the products of singlet exciton fission, a phenomenon in some organic semiconductors which can efficiently convert a high-energy singlet into a pair of triplets of approximately half the energy. Therefore, harvesting triplet energy is an important challenge in organic electronics. This was solved in OLEDs by utilizing molecules with efficient phosphorescence from their triplet states. However, molecular acceptors have proven ineffective in the infrared because of strong, non-radiative dissipation.
Here, we demonstrate direct excitonic energy transfer from ‘dark&’ triplets in the organic semiconductor tetracene to colloidal PbS nanocrystals, successfully harnessing molecular triplet excitons in the near infrared. To prove transfer of triplet excitons from tetracene to PbS nanocrystals, we first measure the excitation spectrum of the tetracene/PbS bilayers and observe that optical excitation of tetracene results in emission from the nanocrystals. We determine the yield of excitons transferred to PbS per photon absorbed by tetracene. We calculate an overall transfer yield of greater than one, proving the involvement of triplet excitons generated by singlet exciton fission in tetracene. We find that the efficiency of energy transfer is consistent with a Dexter mechanism and exponentially dependent on the length of the ligand spacers. The dominance of triplet energy transfer is confirmed using an external magnetic field to vary the rate of singlet exciton fission. By varying the energy of the nanocrystals, we demonstrate that triplet transfer proceeds only when it is energetically downhill. Finally, we characterize the rate of triplet energy transfer using transient photoluminescence.
This demonstration of triplet energy transfer to colloidal nanocrystals suggests a clear path towards several applications. For example, nanocrystals could serve as a transfer layer to help shunt triplet energy from organic materials into conventional solar cells. This could enhance power conversion efficiency by utilizing singlet fission to generate two triplets for every absorbed photon, potentially doubling the photocurrent at short wavelengths. Similarly, if the photoluminescence quantum yield of the colloidal nanocrystals is improved, the combination of nanocrystals and singlet exciton fission should enable the incoherent emission of up to two photons from one absorbed photon, with potential applications including phosphors for high efficiency lighting.

New molecules are being synthesized every week as candidates for organic electronics and yet only a few will make viable devices (i.e OFETs, OLEDs). Even more than the molecular engineering aspects, it is the solid-state morphology of the material, whether in single crystal or thin film applications, that will dictate the mobilities and efficiencies of the device. Studying small molecules with specific variations will give details on how molecular characteristics affect the solid state packing and charge transport properties of organic materials.
The presented work will focus on a series of tripentalene heteroanalogues containing thienothiophene, thienopyrrole, and thiazolothiazole building blocks as organic semiconductors. These crystalline compounds have been designed and synthesized to allow systematic permutations in the heteroatoms, from sulfur to nitrogen. Several polymorphs of the new semiconductors have been isolated, allowing analysis of the relationships between packing characteristics and charge mobilities. Studies involving X-ray crystallography, electrochemistry, spectroscopy, and OFET device analysis provide insights into how the nature and position of heteroatoms effect morphology, charge mobility and solid state luminescent properties. Intermolecular contacts, including S-S and S-N contacts and H-bonding interactions, are analyzed in order to correlate the molecular structure with the supramolecular assembly and the charge transport properties.

Solution processed polymer light-emitting diode (PLED) with a photonic crystal structure was fabricated to enhance the outcoupling efficiency. The outcoupling efficiency of organic light-emitting diodes (OLEDs) has limited the overall efficiency of OLEDs. Waveguide modes in several interfaces in OLEDs could not be extracted to outside due to the index mismatch and total internal reflection (TIR) among these interfaces. To extract these guided modes, some research groups have introduced photonic crystal structures. However, these photonic crystal structures have required an additional planarization layer to prevent uneven deposition of organic materials.
Hence, we coated the solution processed conducting materials which can be act as an anode and a planarization layer, simultaneously, directly on a photonic crystal structured substrate. The photonic crystal structure was fabricated on a quartz substrate using a holographic lithography and reactive ion etching (RIE) with an assistance of chrome hardmask. A two-dimensional reciprocal hole array structure with a period of 400 nm and a depth of hole around 230 nm could be successfully patterned on a quartz substrate. Sol-gel deposited gallium-doped zinc-oxide (GZO) was coated on a nanostructure patterned substrate by spin-coating and subsequent annealing process. By coating this solution processed anode material, flat and well-planarized anode surface on a photonic crystal structured substrate could be obtained. Structured surfaces could not be observed on an anode coated area, and GZO coated region had similar flat surface the GZO films deposited on a planar quartz substrate. PLEDs with a structure of anode, hole injection/transport layer (HITL, PEDOT:PSS), emissive layer (EML, SPG-01T) and cathode (Lithium fluoride and aluminum) were fabricated on a planar quartz substrate and a structured quartz substrate to compare the effectiveness of an anode coated photonic crystal structure in light extraction of PLEDs. Diffracted light emissions from the emission pixels of photonic crystal structure PLEDs could be observed which were not appeared in planar PLED structures. Twofold enhancements in current efficiencies and power efficiencies of PLEDs were obtained by using direct solution coating scheme of anode materials on photonic crystal structure.
This work was supported by the IT R&D program of MOTIE/KEIT [KI002104, Development of Fundamental Technologies for Flexible Combined-Function Organic Electronic Device], Global PH.D Fellowship Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2011-0006795), and the OLED center of Samsung Display and Inter-University Semiconductor Research Center (ISRC) of Seoul National University.

Non-vacuum deposition#12288;techniques has attracted increasing attention for the development of low-cost,#12288;low-energy-consumption, and/or flexible electronic devices with organic semiconductor devices. In our previous study, we demonstrated successful formation of organic semiconductor thin films under atmospheric pressure using a solution-based vapor deposition method, that is, the ultrasonic-assisted mist deposition method#12288;(Mist-CVD). Though this technology is one of solution-based deposition method, the liquid particle size is small and uniform, it is possible to achieve uniform formation of the thin films equal to that of vacuum deposition method.
As hole transport layer of organic LED, N,N&’-Bis(3-methylphenyl)-N,N&’-diphenylbenzidine (TPD) and N,N'-Di(1-naphthyl)-N,N'-diphenylbenzidine (α-NPD) are used which usually used in vacuum evaporation method, we prepared solution of mixture of TPD and α-NPD with butyl acetate. The growth temperature and the growth time of the both materials were controlled. Nitrogen gas was used both as carrier and dilution gases with the flow rates of 1.0 or 2.0, and 1L/min, respectively.
A crystalline film like as deposited by vacuum evaporation method was obtained in both materials. We obtained clear topographic images and surface potential images by simultaneously scanning the same area in vacuum with DFM/KFM using the FM detection method.
We report the details of the surface potential of the hole transport materials of OLEDs fabricated under atmospheric pressure by mist-deposition method.

Solution processed small molecule bulk heterojunction solar cells with power conversion efficiencies (PCE) of 8% have recently been reported. This achievement demonstrates that solar cells fabricated from blends of small molecule donors and fullerene acceptors are a viable alternative to polymer:fullerene based systems. Advantages of using small molecules as donors include the ease of synthesis and purification. Moreover, in contrast to polymers, conjugated small molecules do not suffer from broad molecular weight distributions or batch to batch variations. The recent rise in PCE of small molecule based photovoltaics has largely come due to dramatic rises in the fill factor from previous values less than 50% to upwards of 75% for the best performing systems reported to date. However, the origin of this increase in FF is not well understood and has unfortunately come along with increasingly arduous and costly synthetic procedures.
In this work, we employ single carrier diode measurements to gauge the hole and electron mobilities of over 12 different donor molecules comprising a range of different molecular structures and solar cell PCEs. Both pristine donor molecule films and films blended with [6,6] phenyl-C71-butyric acid methyl ester (PC₇₁BM) are investigated. While in almost all cases efficient electron mobilities in the blend films exceeding 10^-8 m²/Vs are established, the blend film hole mobilities range several orders of magnitude depending on donor materials and processing conditions. A strong correlation between blend film hole mobility and solar cell device FF is observed indicating that 10^-8 m²/Vs stands as the minimum mobility needed to obtain FFs greater than 65%. As neat film hole mobilities are found to always exceed the blend film, a similar threshold value for the pristine film hole mobility emerges which may serve as useful metric for screening new donor materials. Further investigation using x-ray scattering techniques demonstrates the importance of structural order for establishing blend film hole mobilities that can approach the pristine film value. The dependence of mobility on donor-acceptor blend ratio is found to depend strongly on structural order in the pi-pi stacking direction in particular. These findings are supported by the trends in energetic disorder as measured by Kelvin probe. Altogether, these results reveal that the recent rises in FF of small molecule based solar cells can be attributed to the improved hole transport properties achieved in blended films with highly ordered domains. Furthermore, the insights gained from this work will enable the development of the next class molecular donors for efficient photovoltaics that can be readily synthesized in a cost effective manner.

Squaraines are a class of small molecules enthusiastically investigated for organic photovoltaics (OPV) along with other applications such as bio-imaging and photo dynamic therapy. The choice of side group has a surprising influence over the packing and aggregation of these molecules in the solid state active layer of a target device. Two new squaraine molecules with bulky aniline alkyl groups were synthesized with and without stabilizing hydroxyl groups hydrogen bonded to the central C4O2 group. The goal of the work was to understand the relative influence of the hydroxyl groups and R-groups towards the packing as it influences device efficiency. The molecules were characterized for UV-VIS absorption and fluorescence in a variety of blends with PCBM in a variety of different morphologies, controlled through annealing. Changes in device efficiencies were correlated with this spectroscopic data. Spectroscopic data and interpretations from device efficiency changes were also correlated with results from computational chemistry modeling using commercially available software and some simple essential states modeling. We will present an overview of synthesis, characterization, experimental validation of modeling results and device data. We will put forward a description as to how the functionalization of the squaraine influences the efficiency of the device through mechanistic interpretations.

Tandem organic light emitting diodes (OLEDs) which have multiple electroluminescence (EL) units stacked in series with charge generation unit (CGU) attract researchers attention since their low operation current density are capable of achieving high efficiency, luminance and long lifetime. Most important factor for efficient tandem OLEDs is a CGU which is directly related to charge generation and high transparency.
In this study, we investigated improved CGU by applying interfacial 8-Hydroxyqunolinolato-lithium (Liq) layer. We fabricated enhanced CGU consisted in n-type doped electron transport layer (ETL), Liq, thin Aluminium (Al) and hole injection layer (HIL) with 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN). The charge generation films with inserting Liq clearly showed improved charge carrier generation properties compared to the conventional CGUs. After ensuring performance of CGU, we fabricated tandem white OLED applying this CGU with interfacial Liq layer. We employed two highly efficient phosphorescent dyes, iridium(III)[bis(4,6-difluorophenyl)-pyridinato-N,C^2&’]picolinate (FIrpic) for blue emission and iridium(III)[bis(2-phenylbenzothiozolato-N.C^2') accetylacetonate (BT)₂Ir(acac) for yellow emission.
In addition, using atomic force microscopy (AFM), capacitance-Voltage (C-V) measurement and transmittance spectroscopy, we confirmed efficient CGU for tandem whtie OLEDs employing Liq layer. From analysis, improved CGU using interfacial layer had many properties for effective CGU such as lower surface roughness, more charge carrier generation and high transparency compared to conventional CGU. The CGU without Liq exhibited 17.8V driving voltage at 100mA/cm², while the CGU with adopting Liq layer exhibited 15.8V driving voltage at 100mA/cm², reducing 2V in driving voltage. The white tandem OLED with CGU applying Liq layer achieved the external quantum efficiency of 33.0%.

Unlike typical inorganic semiconductors with a crystal structure, the charge dynamics of π-conjugated polymers (π-CPs) are severely limited by the presence of amorphous portions between the ordered crystalline regions. Thus, the formation of interconnected pathways along crystallites of π-CPs is desired to ensure highly efficient charge transport in printable electronics. Here we report the formation of nano-crystallite networks in π-CP films by employing novel template-mediated crystallization (TMC) via polymer-templates or doped carbon nanotube (CNT)-templates. The lateral and vertical charge transport of TMC-treated films increased by two orders of magnitude compared with pristine π-CPs. In particular, because of the unprecedented room temperature and solution-processing advantages of our TMC method using the doped CNT-templates, we achieve a field-effect mobility of “7.0 cm²#8729;V^-1#8729;s^-1” using a plastic substrate, which corresponds to the highest value reported thus far. Because our findings can be applied to various π-conjugated semiconductors, our approach is universal and is expected to yield high-performance printable electronics.

The fabrication of organic light-emitting diodes (OLEDs) by high throughput printing techniques requires the development of solution processable electron injection layers. Today, either alkali salts or low work-function alkaline earth metals like calcium or barium are used as cathode layers in OLEDs. These materials are highly reactive and cannot be easily solution processed but have to be evaporated.
In recent years, different polymers have been investigated as electron injection layers in OLEDs. [1,2] Among others, amino-functionalized polyfluorenes have raised interest due to a vast variety of different materials showing very promising properties when used as electron injection layers in OLEDs. Furthermore, its solubility in highly polar solvents enables these materials to be used in multilayer device architectures. [2]
Solvent-additives are widely used in bulk-heterojunction solar cells in order to optimize the morphology of the active layer. [3]
In this work, we investigate an amino-functionalized polyfluorene as electron injection layer in OLEDs. We demonstrate that its performance can be considerably increased by adding a functionalized alkane to the polyfluorene solution. X-ray photoelectron spectroscopy shows that the polymer thickness decreases with increasing additive concentration which suggests a better packing of the polymers. At the same time, the cathode work-function decreases with increasing additive concentration. These results indicate a change in morphology of the polyfluorene layer. Furthermore, AFM measurements suggest a good homogeneity of the polyfluorene layer independent of additive concentration.
Finally, we solution process OLEDs that use a PPV derivative commonly known as Super-Yellow as emitting layer and the polyfluorene in combination with silver as cathode layer. OLEDs, that use a mixture of functionalized alkane and polyfluorene, exhibit an about 0.5 eV lower turn-on voltage while the maximum luminance is almost doubled compared to OLEDs without additive. Furthermore, operational lifetimes are improved by more than a factor of three.
[1] Stolz et al., ACS Applied Materials and Interfaces, 6:6616-6622, 2014
[2] Huang et al., Chemical society reviews, 39:2500-2521, 2010
[3] Peet et al., Advanced Materials, 21:1521-1527, 2009

The high costs of materials and processing to fabricate state-of-the-art multi-layered small-molecule organic light-emitting diodes (OLEDs) by standard vacuum deposition has been a critical impediment to mass production at a low cost. Although polymer LEDs have good solution processability and multi-functionality, the low luminous efficiencies of polymer LEDs have been a major challenge. Therefore, solution-processed small-molecule OLEDs has been considered as a promising alternative to standard vacuum deposited small-molecule OLEDs. However, the solution-processed small-molecule OLEDs have also suffered from low luminous efficiency and difficulty to multi-layer solution process. Therefore, high efficiency should be achieved in simple-structured small-molecule OLEDs fabricated using a solution process at a low cost. Here, we report extremely efficient solution-processed simple-structured small-molecule OLEDs using novel universal electron-transporting host materials based on tetraphenylsilane with pyridine moieties. These materials have wide band gap, high triplet energy level, and good solution processiblity. We employed self-organized polymeric hole injection layer and mixed host emitting layer to achieve high efficiency in a simple structure without additional hole transporting or electron blocking layers. Our universal host materials provide efficient energy transfer to phosphorescent dopant materials and balanced charge transport in a mixed-host emitting layer. Simplified solution-processed small-molecule orange-red (~97.5 cd/A), green (~101.5 cd/A) and white (~74.2 cd/A) phosphorescent OLEDs exhibit the highest recorded current efficiencies of solution-processed OLEDs reported to date. We demonstrated a solution-processed flexible solid-state-lighting device as a potential application of our materials and devices.

Polymer semiconductor based organic thin film transistors (OTFTs) can be used in radio-frequency identification (RFID) tags, flexible displays, medical sensors, and memory devices, etc.; but their low charge carrier mobilities have limited many applications. Recently, several classes of π-conjugated polymers with electron donor (D) and acceptor (A) units showed highly efficient charge transport performance with field effect mobility higher than 1 cm²V^-1s^-1, which can rival amorphous silicon that has a typical mobility range of 0.1-1 cm²V^-1s^-1. Particularly, diketopyrrolopyrrole (DPP)-based polymers are among the best performing polymer semiconductors for OTFTs to date, and all the high mobility DPP polymers reported are based on five-membered ring-flanked DPP building blocks with high coplanarity.
The first reported phenyl-substituted DPP block (DBP) has a large torsion angle of ~20-40° between the six-membered phenyl ring and the DPP core. Thus the DBP-based polymers showed very low mobility values resulting from the low coplanarity. In this study, we used the 2-pyridinyl units to flank the DPP (DBPy) core to achieve a highly coplanar DBPy building block (with a dihedral angle of 0°), and pyridine is an electron deficient moiety, which would make DBPy a strong electron acceptor. Thus polymers based on DBPy are expected to exhibit efficient electron transport characteristics in OTFTs. Here we report the DBPy based polymer that showed record electron mobility of up to 6.3 cm²V^-1s^-1.
sect; This work is published in Adv. Mater.2014, 26, 2636

Fullerene derivatives, most notably [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM), are the go-to electron acceptors for use in organic solar cells. They have high electron mobility and an appropriate lowest unoccupied molecular orbital (LUMO) energy level that provides proper energy level alignment with a wide array of electron donors and electrode materials. Though a number of non-fullerene electron acceptors have been synthesized having appropriate LUMO energy levels and electron mobilities, none have demonstrated the same kind of universal compatibility with the wide array of donors as fullerene derivatives. We report the design, synthesis and characterization of fluorinated and chlorinated contorted hexabenzocoronenes (HBC) as electron acceptors for solar cells. We found halogenation to decrease both the highest occupied molecular orbital (HOMO) and LUMO energy levels of the parent compound away from the vacuum level; fluorination results in a 60 meV/F shift in both the HOMO and the LUMO. While the HOMO of HBC also decreases by 60 meV/Cl, the decrease in the LUMO level is slightly larger, resulting in a net decrease in the band gap of the chlorinated derivatives compared to the fluorinated counterparts. Disappointingly, none of the fluorinated contorted HBCs are electrically active despite excellent energy level alignment with P3HT and the electrodes employed. The chlorinated derivatives, on the other hand, are electrically active, routinely yielding thin-film transistors with electron mobilities of 10^-2 cm²/Vsec and solar cells with P3HT exceeding 1%. We attribute this dramatic discrepancy in electrical activity between the fluorinated and chlorinated HBCs to subtle differences in which the molecules pack in the solid state.

The ability to control the morphology and crystal structure of organic semiconductors allows their physical properties to be tuned. Desirable characteristics for specific device applications including organic photovoltaic (OPV), thin film transistor (OTFT) and light emitting diode (OLED) geometries can be preferentially selected. To this end, thin organic or inorganic layers have been employed as structural templates for the improvement of device parameters. A prominent example of this methodology is the insertion of a copper (I) iodide (CuI) interlayer between a transparent electrode (indium tin oxide) and a planar phthalocyanine (Pc) active layer to modify molecular orientation in the organic film. The crystal structure of planar Pcs is inherently anisotropic due to face-to-face packing motifs creating short intermolecular distances within stacks and larger inter-stack spacings which impact charge transport and optical absorption in single crystals and thin films. Therefore, controlling the direction along which the molecules have closest contacts can lead to improved device characteristics. The effect of this structural modification has been observed in OPVs and theoretical simulations have been used to model the change in charge transport and absorption profile characteristics in Pc/fullerene devices. Despite this, the nature of the interaction causing this modification has not been studied in any depth. Herein, we use a prototypical CuI / planar Pc system to produce detailed surface and crystal structure information using atomic force microscopy (AFM) and X-ray diffraction (XRD)¹. The out-of-plane structure of the CuI layer is characterised and identified as the (111) plane of single crystal CuI. The dependence of surface morphology and grain size in the CuI (111) templating layer upon substrate temperature is demonstrated. The formation of a thin film of iron phthalocyanine (FePc) on this model layer is characterised at multiple points during growth, changes in the surface morphology were observed, and the crystal structure of the final film is used to infer the molecular orientation therein. These changes are elucidated using the re-determined single crystal structure of FePc, which is also presented. Control of both molecular orientation and grain size in FePc films is demonstrated using this methodology. The behaviour of a non-planar Pc (Vanadyl Phthalocyanine, VOPc) evaporated on to CuI templating layers is also observed and compared to that of FePc. (1. ‘Controlling templating effects at the organic/inorganic interface using (111) oriented copper iodide&’, L. A. Rochford, D. S. Keeble, O. J. Holmes, G. J. Clarkson and T. S. JonesJ. Mater. Chem. C. (accepted 2014))

We use a pump and probe transient grating experiment to investigate excitonic processes in the rubrene single crystal on a time scale up to 10 ns, which allows us to extract unique information that is otherwise not available from either pump and probe experiments limited to the picosecond time scale or from other experiments that follow the photoinduced dynamics on longer time scales. We find that bimolecular interactions affect the decay of the photoinduced signal over a few nanoseconds for photoexcitation densities of the order of ~ 10²⁰ photons/cm³. Near these photoexcitation intensities the decay of the photoinduced excitation follows a typical power law that is due to high-density exciton-exciton interactions. Because of the high efficiency singlet exciton fission observed in rubrene, these bimolecular interactions are likely those between triplet excitons or between coherent quantum superpositions of a singlet and a triplet-exciton pair. We find that half of these photoexcited states disappear after 1 ns for photoexcitation densities of ~ 10²⁰ photons/cm³, corresponding to an average distance of ~ 2 nm between excited states. This data can be used to better characterize the diffusion process that allows the interaction to take place.

Perylene diimides (PDIs) are versatile n-type materials showing great promise in a number of organic semiconductor applications. In organic photovoltaic devices (OPVs), PDIs are being heavily investigated as alternatives to fullerene based acceptors, primarily owing to their strong light absorption in the visible region and ease of synthesis. By derivatizing the perylene core and imide nitrogens, charge transport and light harvesting properties can be altered leading to competitive OPV efficiencies and excellent electron mobilities.
In this talk the synthesis, ultrafast photophysics and semiconducting properties of a series of sulfur-substituted (thionated) PDIs will be presented. Thionation was achieved by replacement of the imide oxygens in one step using commercially available Lawesson&’s reagent. As the degree of thionation increases, the electron affinity is increased resulting in a narrowing of the optical band gap and systematic red shift in optical absorption. Interestingly, thionation renders the PDIs completely non-emissive which is due to sub-picosecond intersystem crossing to triplet states, as determined by ultrafast pump-probe spectroscopy. Remarkably, we find that the rate of intersystem crossing is independent of the number of attached sulfur atoms. Solid state optical properties were determined for each derivative, and show an increase in molecular H-aggregation as the degree of thionation is increased. Bottom gate, bottom contact OFETs employing the PDIs show an increased electron mobility proportional to the degree of sulfur atom substitution, leading to a mobility of 0.18 cm² v^-1 s^-1 in pristine films of the fully thionated analogue. AFM and 2D XRD experiments are used to explain this trend. These results demonstrate the potential of thionation to enhance optoelectronic and materials properties of PDIs, opening up a new pathway for the development of high performance semiconductors based on PDI.